THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 

DAVIS 

GIFT  OF 

THE  PIERCE  FAMILY 


International  Science 


THE     SUN 


BY 

C.   A.   YOUNG,    Ph.D.,   LL  D. 

PROFESSOR  OF  ASTRONOMY  IN  PRINCETON  UNIVERSITY 


WITH  NUMEROUS  ILLUSTRATIONS 


NEW    AND    REVISED    EDITION 


Ube  TKHerner  Company 

JBoofe  flDanufacturera 

Bfcron ,    ©bio 


Copyright  1881,  1886,  1895 
By  D.  Appleton  and  Company 


MADE      BY 

THE    WERNER    COMPANY 
AKRON,     OHIO 


PKEFACE  TO   THE  EEYISED   EDITION. 


SINCE  the  original  publication  of  this  book  in  1881 
great  advances  have  been  made  in  our  knowledge  of  the 
sun,  and  in  the  four  or  five  editions  which  have  subse- 
quently appeared  the  attempt  has  been  made  to  keep 
the  book  measurably  up  to  date  by  the  addition  of 
appendices  and  notes. 

The  time  has  come,  however,  when  such  expedients 
are  no  longer  adequate,  and  the  author  has  therefore 
thoroughly  revised  the  work,  rewriting  portions,  em- 
bodying notes  in  the  text,  and  adding  whatever  seemed 
necessary  to  make  the  book  fairly  representative  of  the 
solar  science  of  to-day. 

The  progress  of  discovery  with  respect  to  helium 
has  been  so  continuous  and  rapid  during  the  revision 
and  printing  of  the  work,  that  I  have  found  it  neces- 
sary to  append  a  supplementary  note  upon  the  subject. 

Special  thanks  are  due  to  Prof.  Hale  for  several  of 
the  finest  of  the  twenty  new  illustrations,  and  to  Ginn 
and  Co.  for  the  use  of  one  or  two  cuts  from  my  Gen- 
eral Astronomy. 

November,  1895. 


FROM  PREFACE  TO  THE  FIRST  EDITION. 


IT  is  my  purpose  in  this  little  book  to  present  a  gen- 
eral view  of  what  is  known  and  believed  about  the  sun, 
in  language  and  manner  as  unprofessional  as  is  con- 
sistent with  precision.  I  write  neither  for  scientific 
readers  as  such,  nor,  on  the  other  hand,  for  the  masses, 
but  for  that  large  class  in  the  community  who,  without 
being  themselves  engaged  in  scientific  pursuits,  yet 
have  sufficient  education  and  intelligence  to  be  inter- 
ested in  scientific  subjects  when  presented  in  an  un- 
technical  manner ;  who  desire,  and  are  perfectly  com- 
petent, not  only  to  know  the  results  obtained,  but  to 
understand  the  principles  and  methods  on  which  they 
depend,  without  caring  to  master  all  the  details  of  the 
investigation. 

I  have  tried  to  keep  distinct  the  line  between  the 
certain  and  the  conjectural,  and  to  indicate  as  far  as 
possible  the  degree  of  confidence  to  be  placed  in  data 
and  conclusions. 

It  is  hardly  necessary  to  say  that  the  work  has  small 
claims  to  originality.  I  have  made  use  of  material 

suited  to  my  purpose  from  all  accessible  sources ;  possi- 

rii 


Viii  FROM  PREFACE   TO   THE  FIRST  EDITION. 

bly  in  some  cases  (though  I  hope  not)  without  giving 
sufficient  credit  to  the  original  authority.  I  have  been 
specially  indebted  to  Secchi,  Lockyer,  Proctor,  Ran- 
yard,  Vogel,  Schellen,  and  Langley.  .  .  . 

PRINCETON,  August  1,  1881. 


CONTENTS. 


INTRODUCTION. 

PAGE 

The  Sun's  Relation  to  Life  and  Activity  upon  the  Earth.— Brief  State- 
ment of  the  Principal  Facts  relating  to  the  Sun,  and  of  the  Ac- 
cepted Views  as  to  its  Constitution 1 


CHAPTER  I. 
DISTANCE  AND  DIMENSIONS   OF  THE  SUN. 

Importance  of  the  Problem. — Definition  of  Parallax. — Aristarchus's 
Determination  of  the  Parallax. — Different  Available  Methods. — 
Observations  of  Mars  and  of  the  nearer  Asteroids. — Transits  of 
Venus.  —  Observations  of  Contacts,  Heliometric  Measures,  and 
Photographic  Work. — Determination  of  Solar  Parallax  by  means 
of  the  Velocity  of  Light ;  by  Lunar  and  Planetary  Perturbations. — 
Illustrations  of  the  Immensity  of  the  Sun's  Distance. — Diameter 
of  the  Sun.— The  Sun's  Mass  and  Density 


CHAPTER  II. 

METHODS  AND   APPARATUS  FOR   STUDYING    THE  SUR- 
FACE OF  THE  SUN. 

Projection  of  Solar  Image  upon  a  Screen. — Carrington's  Method  of 
determining  the  Position  of  Objects  on  Sun's  Surface. — Solar 
Photography.  — Photohel iographs  —  Cornu's  Methods.— Telescope 
with  Silvered  Object-Glass.— Herschel's  Solar  Eyepiece.— The 
Polarizing  Eyepiece  .  .  .  ...  .;£>  ....  .  43 


x  CONTENTS. 

CHAPTER  III. 
THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM. 

PAGE 

The  Spectrum  and  Fraunhofer's  Lines. — The  Prismatic  Spectroscope ; 
Description  of  Various  Forms  and  Explanation  of  its  Operation. — 
The  Diffraction  Spectroscope. — The  Concave  Grating. — Analyzing 
and  Integrating  Spectroscopes. — The  Telespectroscope  and  its 
Adjustment. — The  Spectrograph. — Explanation  of  Lines  in  the 
Spectrum. — Kirchhoff's  Kesearches  and  Laws. — The  Sun's  Ab- 
sorbing Atmosphere  and  Reversing  Layer. — Elements  present  in 
the  Sun. — Lockyer's  Researches  and  Hypothesis. — Basic  Lines. — 
Dr.  H.  Draper's  Investigations  as  to  the  Presence  of  Oxygen  in 
the  Sun. — Schuster's  Observations. — Effect  of  Motion  upon  Wave- 
Length  of  Rays  and  Spectroscopic  Determinations  of  Motion  in 
Line  of  Sight 5? 


CHAPTER  IV. 
SUN-SPOTS  AND  THE  SOLAR  SURFACE. 

Granulation  of  Solar  Surface. — Views  of  Langley,  Nasmyth,  Secchi, 
and  others. — Faculse. — Nature  of  the  Photosphere. — Janssen's 
Photographs  of  Solar  Surface — the  Reseau  Photospherique. — Dis- 
covery of  Sun-Spots. — General  Appearance  and  Structure  of  a 
Spot. — Its  Formation  and  Disappearance. — Duration  of  Sun-Spots. — 
Remarkable  Phenomena  observed  by  Carrington  and  Hodgson. — 
Observations  of  Peters. — Dimensions  of  Spots. — Proof  that  Spots 
are  Cavities.— Sun-Spot  Spectrum.—"  Veiled  Spots."— Rotation  of 
Sun.— Equatorial  Acceleration.— Explanations  of  the  Accelera- 
tion.— Position  of  Sun's  Axis  and  Secchi's  Table  for  its  Position 
Angle  at  Different  Times  in  the  Year.— Proper  Motions  of  Spots.— 
Distribution  of  Spots 102 


CHAPTER  V. 

PERIODICITY  OF  SUN-SPOTS;  THEIR  EFFECTS  UPON  THE 
EARTH,  AND  THEORIES  AS  TO  THEIR  CAUSE  AND  NA- 
TURE. 

Observations  of  Schwabe. — Wolf's  Numbers. — Proposed  Explanations 
of  Periodicity. — Connection  between  Sun-Spots  and  Terrestrial 
Magnetism.  —  Remarkable  Solar  Disturbances  and  Magnetic, 


CONTENTS. 


XI 

PAGE 


Storms.— Effect  of  Sun- Spots  on  Temperature.— Sun-Spots,  Cy- 
clones, and  Kainfall. — Researches  of  Symons  and  Meldrum. — Sun- 
Spots  and  Commercial  Crises. — Galileo's  Theory  of  Spots. — Her- 
schel's  Theory. — Secchi's  First  Theory. — Zollner's. — Faye's.— 
Secchi's  Later  Opinions. — Theories  of  Lockyer,  Schaeberle,  Oppol- 
zer,  and  others  .  .  .  ...  .  .  .  .  .  151 


CHAPTER  VI. 
THE  CHROMOSPHERE  AND  THE  PROMINENCES. 

Early  Observations  of  Chromosphere  and  Prominences. — The  Eclipses 
of  1842, 1851,  and  I860.— The  Eclipse  of  1868.— Discovery  of  Jans- 
sen  and  Lockyer. — Arrangement  of  Spectroscope  for  Observations 
upon  Chromosphere. — Spectrum  of  Chromosphere. — Lines  always 
present. — Lines  often  reversed. — Spectrographs  of  Hale  and  Des- 
landres. — Motion  Forms. — Double  Reversal  of  Lines. — Distribution 
of  Prominences.— Magnitude. — Classification  of  Prominences  as 
quiescent,  and  eruptive  or  metallic. — Isolated  Clouds. — Violence 
of  Motion. — Observations  of  August  5, 1872. — Theories  as  to  the 
Formation  and  Causes  of  the  Prominences  .  192 


CHAPTER  VII. 
THE   CORONA. 

General  Appearance  of  the  Phenomenon. — Various  Representations. — 
Eclipses  of  1857, 1860, 1867, 1868,  1869, 1871,  and  1878.— Proof  that 
the  Corona  is  mainly  a  Solar  Phenomenon. — Brightness  of  the 
Corona. — Connection  with  Sun-Spot  Period. — Spectrum  of  the 
Corona. — Application  of  the  Analyzing  and  Integrating  Spectro- 
scopes.— Polarization. — Evidence  of  the  Slitless  Spectroscope  as 
to  the  Constitution  of  the  Corona. — Changes  and  Motions  in  the 
Corona. — Its  Form  and  Constitution,  and  Theories  as  to  its  Nature 
and  Origin '....-;  .  .237 


CHAPTER  VIII. 
THE  SUXTS  LIGHT  AND  HEAT. 

Sunlight  expressed  in  Candle- Power.— Method  of   Measurement— 
Brightness  of  the  Sun's  Surface.— Langley's  Experiment.— Dimin- 


xii  CONTENTS. 


ution  of  Brightness  at  Edge  of  the  Sun's  Disk. — Hastings's  View 
as  to  Nature  of  the  Absorbing  Envelope. — Total  Amount  of  Ab- 
sorption by  Sun's  Atmosphere. — Thermal,  Luminous,  and  Actinic 
Kays :  their  Fundamental  Identity  and  Differences. — Measurement 
of  the  Sun's  Eadiation. — Herschel's  Method. — Expressions  for  the 
Amount  of  Sun's  Heat. — Pouillet's  Pyrheliometer. — Crova's. — 
Violle's  Actinometer. — Langley's  Eesearches. — Absorption  of  Heat 
by  Earth's  Atmosphere  :  by  the  Sun's. — Question  as  to  Differences 
of  Temperature  on  Different  Portions  of  Sun's  Disk. — Question  as 
to  Variation  of  Sun's  Eadiation  with  Sun-Spot  Period.— The  Sun's 
Temperature — Actual — Effective.— Views  of  Secchi,  Ericsson,  Pou- 
illet,Vicaire,  Eosetti,  Le  Chatelier,  and  Wilson  and  Gray. — Evidence 
from  the  Burning-Glass. — Langley's  Experiment  with  the  Besse- 
mer "  Converter." — Permanency  of  Solar  Heat  for  last  Two  Thou- 
sand Years.— Meteoric  Theory  of  Sun's  Heat.— Helmholtz's  Con- 
traction Theory. — Possible  Past  and  Future  Duration  of  the  Sun's 
Supply  of  Heat. — Siemens's  Untenable  Theory  ....  276 


CHAPTEE  IX. 

SUMMARY  OF  FACTS,  AND  DISCUSSION  OF  THE  CONSTI- 
TUTION OF  THE  SUN. 

Table  of  Numerical  Data. — Constitution  of  Sun's  Nucleus. — Peculiar 
Properties  of  Gases  under  High  Temperature  and  Pressure. — 
Characteristic  Differences  between  a  Liquid  and  a  Gas. — Consti- 
tution of  the  Photosphere  and  Higher  Eegions  of  the  Sun's  At- 
mosphere.— Professor  Hastings's  Theory. — Pending  Problems  of 
Solar  Physics 828 

NOTE  ON  HELIUM 844 

INDEX  .  .   363 


THE    SUN. 


INTKODUCTIOK 

THE  SUN^S  RELATION  TO  LIFE  AND  ACTIVITY  UPON  THE  EARTH. 

Brief  Statement  of  the  Principal  Facts  relating  to  the  Sun,  and  of  the 
Accepted  Views  as  to  its  Constitution. 

IT  is  true  that  from  the  highest  point  of  view  the 
sun  is  only  one  of  a  multitude — a  single  star  among 
millions — thousands  of  which,  most  likely,  exceed  him 
in  brightness,  magnitude,  and  power.  He  is  only  a  pri- 
vate in  the  host  of  heaven. 

But  he  alone,  among  the  countless  myriads,  is  near 
enough  to  affect  terrestrial  affairs  in  any  sensible  degree ; 
and  his  influence  upon  them  is  such  that  it  is  hard  to 
find  the  word  to  name  it ;  it  is  more  than  mere  control 
and  dominance.  He  does  not,  like  the  moon,  simply 
modify  and  determine  certain  more  or  less  important 
activities  upon  the  surface  of  the  earth,  but  he  is  almost 
absolutely,  in  a  material  sense,  the  prime  mover  of  the 
whole.  To  him  we  can  trace  directly  nearly  all  the 
energy  involved  in  all  phenomena,  mechanical,  chemi- 
cal, or  vital.  Cut  off  his  rays  for  even  a  single  month, 
and  the  earth  would  die ;  all  life  upon  its  surface  would 
cease. 

There  always  has  been  a  more  or  less  distinct  recog- 
nition of  this  fact.  The  first  man's  experience  of  the 
2 


2  INTRODUCTION. 

first  sunset  ever  witnessed  by  human  eyes  must  have 
made  it  tremendously  obvious,  when  he  saw  the  sun 
descend  below  the  horizon,  and  the  darkness  close  in 
upon  the  earth,  and  felt  the  chill  of  night,  and  fell 
asleep  not  knowing  of  a  sunrise  to  come — unless,  per- 
haps, some  divine  revelation  took  pity  on  the  hopeless 
terror  he  must  otherwise  have  suffered,  or  unless  he 
may  have  been,  like  a  little  child,  slow  to  notice  and 
unable  to  comprehend  what  would  frighten  a  more  in- 
telligent being. 

But  while  the  material  supremacy  of  the  sun  has 
always  been  recognized  by  thoughtful  minds,  and  has 
even  been  made  the  foundation  of  religious  systems,  as 
with  the  Persians,  it  has  been  reserved  for  more  mod- 
ern times,  and  to  our  own  century,  to  show  clearly  just 
how,  in  what  sense,  and  how  far  the  sunbeams  are  the 
life  of  the  earth,  and  the  sun  himself  the  symbol  and 
vicegerent  of  the  Deity.  The  two  doctrines  of  the  corre- 
lation of  forces  and  the  conservation  of  energy,  having 
once  been  distinctly  apprehended  and  formulated,  it 
has  been  comparatively  easy  to  confirm  them  by  experi- 
ment and  observation,  and  then  to  trace,  one  by  one,  to 
their  solar  origin,  the  different  classes  of  energy  which 
present  themselves  in  terrestrial  phenomena — to  show, 
for  instance,  how  the  power  of  waterfalls  is  only  a  trans- 
formation of  the  sun's  heat ;  and  that  the  same  thing 
is  true,  a  little  more  remotely  but  just  as  certainly,  of 
the  power  of  steam,  of  electricity,  and  even  of  animals. 
The  idea  is  now  so  familiar  that  it  is  hardly  necessary 
to  dwell  upon  it,  and  yet,  for  some  of  our  readers  at 
least,  it  may  be  worth  while  to  examine  it  a  little  more 
closely. 

Whenever  work  is  done,  it  is  by  the  undoing  of  some 
previous  work.  When  a  clock  moves,  it  is  the  unwind- 


INTRODUCTION.  3 

ing  of  a  spring  or  the  falling  of  a  weight  which  keeps 
it  going,  and  some  one  must  have  wound  it  up  to  begin 
with.  If  the  water  of  a  river  falls  year  after  year  over 
a  cataract,  and  is  intercepted  to  drive  our  mill-wheels, 
the  river  continues  to  run  because  some  power  is  con- 
tinually raising  and  returning  to  the  hill-tops  the  water 
which  has  flowed  into  the  sea — a  process  precisely 
equivalent  to  the  daily  rewinding  of  the  clock.  If  the 
powder  in  a  rifle  explodes  and  drives  out  the  bullet,  its 
explosive  energy  depends  upon  the  fact  that  some  power 
has  placed  the  component  molecules  in  such  relations 
that,  when  the  trigger  is  pulled,  and  the  exciting  spark 
has,  so  to  speak,  cut  the  bonds  which  hold  them  apart, 
they  rush  together  just  as  suspended  weights  would 
fall  if  freed.  Before  the  same  substance,  which  once 
was  a  charge  of  gunpowder,  but  now  is  dust  and  gas, 
can  again  do  the  same  work,  the  products  of  the  ex- 
plosion must  by  some  power  be  decomposed,  and  the 
atoms  replaced  in  the  same  relations  as  before  the  firing 
of  the  gun ;  and  this  process  is  mechanically  analogous 
to  the  lifting  of  fallen  weights  and  placing  them  upon 
elevated  shelves,  or  hanging  them  from  hooks,  ready  to 
drop  again  when  the  occasion  may  require. 

Precisely  the  same  thing  is  true  of  the  heat  pro- 
duced by  the  combustion  of  ordinary  fuel :  it  is  due  to 
the  collapse  of  molecules,  for  the  most  part  of  oxygen 
on  one  side,  and  carbon  and  hydrogen  on  the  other, 
which  have  been  separated  and  built  up  into  structures 
by  the  action  of  some  laboring  power. 

The  same  can  be  said  of  animal  power,  for  all  inves- 
tigation goes  to  show  that  in  a  mechanical  sense  the 
body  of  an  animal  is  only  a  very  ingenious  and  effective 
machine,  by  means  of  which  the  living  inhabitant  which 
controls  it  can  utilize  the  energy  derived  from  the  food 


4  INTRODUCTION. 

taken  into  the  stomach.  The  body,  regarded  as  a  mech- 
anism, is  only  a  food-engine  in  which  the  stomach  and 
lungs  stand  for  the  furnace  and  boiler  of  a  steam-engine, 
the  nervous  system  for  the  valve-gear,  and  the  muscles 
for  the  cylinder.  How  the  personality  within,  which 
wills  and  acts,  is  put  into  relation  with  this  valve-gear, 
so  as  to  determine  the  movements  of  the  body  it  re- 
sides in,  is  the  inscrutable  mystery  of  life ;  the  facts 
in  the  case,  however,  being  no  less  facts  because  inex- 
plicable. 

And  now,  when  we  come  to  inquire  for  the  source 
of  the  energy  which  lifts  the  water  from  the  sea  to  the 
mountain-top,  which  decomposes  the  carbonic  acid  of 
the  atmosphere,  and  plant-foods  of  the  soil,  and  builds 
up  the  hydrocarbons  and  other  fuels  of  animal  and 
vegetable  tissue,  we  find  it  always  mainly  in  the  solar 
rays.  I  say  mainly  because,  of  course,  the  light  and 
heat  of  the  stars,  the  impact  of  meteors,  and  the  prob- 
able slow  contraction  of  the  earth,  are  all  real  sources  of 
energy,  and  contribute  their  quota.  But,  as  compared 
with  the  energy  derived  from  the  sun,  their  total 
amount  is  probably  something  like  the  ratio  of  starlight 
to  sunlight ;  *  so  small  that  it  is  quite  clear,  as  we  said 

*  Pouillet,  about  1838,  came  to  a  conclusion  entirely  inconsistent 
with  the  statement  of  the  text.  From  his  actinometric  observations, 
he  deduced  a  temperature  of  —224°  F.  (—142°  C.)  for  the  "tem- 
perature of  space,"  which  is  236°  (131  C.)  above  the  absolute  zero.  To 
maintain  this  temperature  of  —  224°,  he  calculated  that  the  stars  and 
space  in  general  must  furnish  to  the  earth  about  85  per  cent,  as  much 
heat  as  the  sun  supplies.  His  calculations,  however,  rest  upon  assump- 
tions as  to  the  laws  of  cooling  and  radiation  which  are  not  at  present  re- 
ceived as  accurate,  and  he  fails  to  take  proper  account  of  the  influence  of 
water-vapor  in  the  air — an  influence,  the  magnitude  of  which  was  first 
brought  out  more  than  twenty  years  later  by  the  researches  of  Tyndall 
and  Magnus.  It  is  now  generally  admitted,  therefore,  that  his  result  can 
not  be  accepted. 


INTRODUCTION.  5 

before,  that  a  month's  deprivation  of  the  solar  rays 
would  involve  the  utter  destruction  of  all  activity  upon 
the  earth. 

It  is  natural,  therefore,  that  modern  science  should 
make  much  of  the  sun,  and  that  the  study  of  solar  phe- 
nomena and  relations  should  be  pursued  with  the  great- 
est interest.  For  the  last  fifty  years  this  has  been 
especially  the  case :  Schwabe's  discovery  of  the  perio- 
dicity of  the  sun-spots  in  1851 ;  the  development  of 
spectroscopic  analysis  between  1854  and  1870;  the 
eclipse  observations  since  1860 ;  the  researches  of  Car- 
rington,  Huggins,  De  La  Rue,  Lockyer,  Janssen,  Secchi, 
Yogel,  Langley,  Hale,  and  others ;  the  establishment  of 
the  observatories  at  Potsdam  and  Meudon — these  are 
all  evidences  of  the  ardor  with  which  astronomers  have 
devoted  themselves  to  the  problems  of  solar  science,  and 
of  their  rich  rewards. 

It  may  be  well,  before  entering  upon  the  more 
extended  discussion  of  our  subject,  to  summarize  here 
a  few  of  the  more  important  and  obvious  facts  re- 
lating to  the  sun,  with  a  brief  statement  of  the  views 
at  present  generally  held  in  regard  to  its  constitu- 
tion. 

To  the  few  unaided  eyes  which  are  able  to  bear 
its  brilliance  without  flinching,  the  sun  presents  the 
appearance  of  a  round,  white  disk,  a  little  more  than 
half  a  degree  in  diameter — i.  e.,  a  row  of  seven  hundred 
suns,  side  by  side,  would  just  about  fill  up  the  circle  of 
the  horizon.  Usually,  without  a  telescope,  the  surface 
appears  simply  uniform,  except  that  there  is  a  slight 
darkening  at  the  edge,  and  that  once  in  a  while  black 
spots  are  seen  upon  the  disk.  There  is  nothing  in  the 
sun's  appearance  to  indicate  his  real  distance,  and,  until 
that  is  known,  of  course  no  conclusion  can  be  arrived  at 


6  INTRODUCTION. 

as  to  his  true  dimensions ;  but  the  heat  of  his  rays  is 
obvious,  and,  long  before  the  days  of  telescopes  and 
thermometers,  led  to  the  conclusion  that  he  is  nothing 
more  or  less  than  an  enormous  ball  of  fire. 

If  we  watch  him  from  day  to  day  through  the  year, 
beginning  about  the  21st  of  March,  we  shall  find  that 
at  noon  he  daily  rises  higher  in  the  heavens,  until  about 
the  22d  of  June ;  at  this  time  he  ascends  to  the  same 
height  each  noon  for  several  successive  days,  and  then 
slides  slowly  south,  passing  on  September  22d  the  ele- 
vation he  had  at  starting,  and  keeping  on  until,  on  De- 
c-ember 21st,  he  attains  his  farthest  southing  ;  thence  he 
returns,  till  he  reaches  the  place  of  beginning,  and 
night  and  day  again  are  equal. 

If,  at  the  same  time,  one  has  noticed  the  stars  each 
night,  he  will  find  the  constellations  to  have  shifted 
with  the  months,  in  such  a  way  that  it  is  clear  that  the 
sun  has  been  traveling  eastward  among  them  through 
the  sky,  as  well  as  swinging  north  and  south ;  moving, 
in  fact,  yearly  around  the  heavens  in  a  path  which  is  a 
great  circle  of  the  sphere,  inclined  some  23-J0  to  the 
equator,  and  called  the  ecliptic,  because  it  is  only  when 
the  moon  is  near  this  line  at  new  or  full  that  eclipses 
happen. 

There  is  nothing  in  this  motion  which  of  itself  can 
inform  us  whether  its  cause  is  a  real  movement  of  the 
sun  around  the  earth,  or  of  the  earth  around  the  sun. 
At  present,  of  course,  every  one  knows  that  the  earth 
is  really  the  moving  body.  A  careful  watching  shows 
that  her  path  is  not  quite  circular,  or,  at  least,  that  the 
sun  is  not  exactly  in  the  center,  since  it  is  one  hundred 
and  eighty-six  days  through  the  summer  from  the  ver- 
nal to  the  autumnal  equinox,  and  only  one  hundred  and 
seventy-nine  from  the  autumnal  to  the  vernal. 


INTRODUCTION.  f 

This  much  was  known  to  the  ancients,  and  the  one 
additional  fact  that  the  sun's  distance  is  many  times 
greater  than  that  of  the  moon ;  it  is  all  that  could  pos- 
sibly be  learned  without  the  use  of  the  telescope  and 
instruments  of  precision. 

Modern  astronomy  has  gone  much  further.  "We 
now  know  that  the  sun's  average  distance  from  the 
earth  is  about  93,000,000  miles,  and  consequently  that 
his  diameter  is  about  865,000  miles.  The  sun  has 
been  weighed  against  the  earth  and  found  to  contain 
a  quantity  of  matter  nearly  330,000  times  as  great,  and 
comparing  this  with  his  enormous  bulk,  it  appears  that 
his  mean  density  is  only  about  one  fourth  that  of 
the  earth,  or  one  and  a  quarter  times  that  of  water 
— in  other  words,  the  mass  of  the  sun  is  about  one 
fourth  greater  than  that  of  a  globe  of  water  of  the  same 
size. 

The  visible  surface  of  the  sun  has  been  named  the 
photosphere,  and  by  watching  the  spots,  which  occa- 
sionally appear  upon  it,  we  have  ascertained  that  it 
revolves  upon  its  axis  once  in  about  twenty-five  and  a 
quarter  days.  At  times  of  total  eclipse,  when  the  moon 
hides  from  us  the  body  of  the  sun,  we  are  enabled  to 
see  certain  outlying  phenomena  at  other  times  invisible. 
We  find  close  around  the  luminous  surface  a  rose-col- 
ored stratum  of  gaseous  matter  to  which  Frankland  and 
Lockyer  some  years  ago  assigned  the  name  of  chromo- 
sphere. Here  and  there  great  masses  of  this  chromo- 
spheric  matter  rise  high  above  the  general  level  like 
clouds  of  flames,  and  are  then  known  as  prominences  or 
protuberances. 

Outside  of  the  chromosphere  is  the  mysterious  co- 
rona, an  irregular  halo  of  faint,  pearly  light,  composed 
for  the  most  part  of  radial  filaments  and  streamers, 


8  INTRODUCTION. 

which  extend  outward  from  £he  sun  to  an  enormous 
distance ;  often  more  than  a  million  of  miles. 

The  spectroscope  informs  us  that,  in  great  part  at 
least,  the  elements,  which  exist  in  the  lower  regions  of 
the  solar  atmosphere  in  the  state  of  vapor,  are  metals 
we  are  familiar  with  upon  the  earth ;  while  it  shows 
the  chromosphere  and  prominences  to  consist  mainly 
of  hydrogen  and  helium,  and  makes  it  possible  to  ob- 
serve them  even  when  the  sun  is  not  hidden  by  the 
moon.  The  secret  of  the  corona  it  fails  to  unlock  as 
yet,  though  it  informs  us  of  the  presence  in  it  of  an 
unknown  gas  of  inconceivable  tenuity. 

The  pyrheliometer  and  actinometer  measure  for  us 
the  outflow  of  solar  heat,  and  show  us  that  the  blaze  is 
at  least  seven  or  eight  times  as  intense  as  that  of  any 
furnace  known  to  art.  The  quantity  of  heat  emitted  is 
enough  to  melt  a  shell  of  ice  more  than  a  foot  thick 
over  the  whole  surface  of  the  sun  every  second  of  time  : 
this  is  equivalent  to  the  consumption  of  a  layer  of  the 
best  anthracite  coal  over  five  inches  thick  every  single 
minute. 

Combining  the  facts  just  stated,  astronomers  are  for 
the  most  part  agreed  upon  the  following  conclusions  as 
to  the  constitution  of  the  sun  : 

1.  The  central  portion  is  probably  for  the  most  part 
a  mass  of  intensely  heated  gases. 

2.  The  photosphere  is  a  shell  of  luminous  clouds, 
formed  by  the  cooling  and  condensation  of  the  conden- 
sible  vapors  at  the  surface,  where  exposed  to  the  cold 
of  outer  space. 

3.  The  chromosphere  is  composed  mainly  of  uncon- 
densible  gases  (conspicuously  hydrogen)  left  behind  by 
the  formation  of  the  photospheric  clouds,  and  bearing 
something  the  same  relation  to  them  that  the  oxygen 


INTRODUCTION.  9 

and  nitrogen  of  our  own  atmosphere  do  to  our  own 
clouds. 

4,  The  corona  as  yet  has  received  no  explanation 
which  commands  universal  assent.  It  is  certainly  truly- 
solar  to  some  extent,  and  very  possibly  may  be  also  to 
some  extent  meteoric 


CHAPTER  1. 

DISTANCE  AND  DIMENSIONS  OF  THE  SUN. 

Importance  of  the  Problem. — Definition  of  Parallax. — Aristarchus's  Deter- 
initiation  of  the  Parallax. — Different  Available  Methods. — Observa- 
tions of  Mara  and  of  the  nearer  Asteroids. — Transits  of  Venus. — 
Observations  of  Contacts  and  Photographic  Work. — Determination 
of  Solar  Parallax  by  means  of  the  Velocity  of  Light ;  by  Lunar  and 
Planetary  Perturbations. — Illustrations  of  the  Immensity  of  the 
Sun's  Distance. — Diameter  of  the  Sun. — The  Sun's  Mass  and  Density. 

THE  problem  of  finding  the  distance  of  the  sun  is 
one  of  the  most  important  and  difficult  presented  by 
astronomy.  Its  importance  lies  in  this,  that  this  dis- 
tance— the  radius  of  the  earth's  orbit — is  the  base-line 
by  means  of  which  we  measure  every  other  celestial 
distance,  excepting  only  that  of  the  moon  ;  so  that  error 
in  this  base  propagates  itself  in  all  directions  through 
all  space,  affecting  with  a  corresponding  proportion  of 
falsehood  every  measured  line — the  distance  of  every, 
star,  the  radius  of  every  orbit,  the  diameter  of  every 
planet. 

Our  estimates  of  the  masses  of  the  heavenly  bodies 
also  depend  upon  a  knowledge  of  the  sun's  distance 
from  the  earth.  The  quantity  of  matter  in  a  star  or 
planet  is  determined  by  calculations  whose  fundamental 
data  include  the  distance  between  the  investigated  body 
and  some  other  body  whose  motion  is  controlled  or 
modified  by  it ;  and  this  distance  generally  enters  into 
the  computation  by  its  cube,  so  that  any  error  in  it  in- 


DISTANCE  AND  DIMENSIONS  OP  THE  SUN.  H 

volves  a  more  than  threefold  error  in  the  resulting  mass. 
An  uncertainty  of  one  per  cent,  in  the  sun's  distance 
implies  an  uncertainty  of  more  than  three  per  cent,  in 
every  celestial  mass  and  every  cosmical  force. 

Error  in  this  fundamental  element  propagates  itself 
in  time  also,  as  well  as  in  space  and  mass.  That  is  to 
say,  our  calculations  of  the  mutual  effects  of  the  planets 
upon  each  other's  motions  depend  upon  an  accurate 
knowledge  of  their  masses  and  distances.  By  these 
calculations,  were  our  data  perfect,  we  could  predict  for 
all  futurity,  or  reproduce  for  any  given  epoch  of  the 
past,  the  configurations  of  the  planets  and  the  con- 
ditions of  their  orbits,  and  many  interesting  problems 
in  geology  and  natural  history  seem  to  require  for  their 
solution  just  such  determinations  of  the  form  and  po- 
sition of  the  earth's  orbit  in  by-gone  ages. 

Now,  the  slightest  inaccuracy  in  the  data,  though 
hardly  affecting  the  result  for  epochs  near  the  present, 
leads  to  error  which  accumulates  with  the  lapse  of 
time ;  so  that  even  the  present  uncertainty  of  the  sun's 
distance,  small  as  it  is,  renders  precarious  all  conclu- 
sions from  such  computations  when  the  period  is  ex- 
tended more  than  a  few  hundred  thousand  years.  If, 
for  instance,  we  should  find  as  the  result  of  calcula- 
tion with  the  received  data,  that  two  millions  of  years 
ago  the  eccentricity  of  the  earth's  orbit  was  at  a  maxi- 
mum, and  the  perihelion  so  placed  that  the  sun  was 
nearest  during  the  northern  winter  (a  condition  of 
affairs  which  it  is  thought  would  produce  a  glacial 
epoch  in  the  southern  hemisphere),  it  might  easily 
happen  that  our  results  would  be  exactly  contrary  to 
the  truth,  and  that  the  state  of  affairs  indicated  did 
not  occur  within  ten  thousand  years  of  the  specified 
date — and  all  because  in  our  calculation  the  sun's  dis- 


12  THE  SUN. 

tance,  or  the  solar  parallax  by  which  it  is  measured, 
was  assumed  half  of  one  per  cent,  too  great  or  too 
small.  In  fact,  this  solar  parallax  enters  into  almost 
every  kind  of  astronomical  computations,  from  those 
which  deal  with  stellar  systems  and  the  constitution  of 
the  universe,  to  those  which  have  for  their  object  noth- 
ing higher  than  the  prediction  of  the  moon's  place  as 
a  means  of  finding  the  longitude  at  sea. 

Of  course,  it  hardly  need  be  said  that  its  determina- 
tion is  the  first  step  to  any  knowledge  of  the  dimensions 
and  constitution  of  the  sun  itself. 

This  "  parallax  "  of  the  sun  is  simply  the  angular 
semi-diameter  of  the  earth  as  seen  from  the  sun  •  or,  it 
may  be  defined  in  another  way  as  the  angle  between 
the  direction  of  the  sun  ideally  observed  from  the  center 
of  the  earth,  and  its  actual  direction  as  seen  from  a  sta- 
tion where  it  is  just  rising  above  the  horizon. 

"We  know  with  great  accuracy  the  dimensions  of  the 
earth.  Its  mean  equatorial  radius,  according  to  Hark- 
ness's  latest  determination  (agreeing,  however,  very 
closely  with  previous  ones),  is  3963'124  English  miles 
[637T972  kilometres],  and  the  error  can  hardly  amount 
to  more  than  2S $00  of  the  whole — perhaps,  800  feet 
one  way  or  the  other.  Accordingly,  if  we  know  how 
large  the  earth  looks  from  any  point,  or,  to  speak  tech- 
nically, if  we  know  the  parallax  of  the  point,  its  dis- 
tance can  at  once  be  found  by  a  very  easy  calculation : 
it  equals  simply  [206,265  *  X  the  radius  of  the  earth]  -j- 
[the  parallax  in  seconds  of  arc]. 

*  This  number  206,265  is  the  length  of  the  radius  of  a  circle  ex- 
pressed in  seconds  of  its  circumference.  A  ball  one  foot  in  actual  diam- 
eter would  have  an  apparent  diameter  of  one  second  at  a  distance  of 
206,265  feet,  or  a  little  more  than  39  miles.  If  its  apparent  diameter 
were  10",  its  distance  would,  of  course,  be  only  -fa  as  great. 


DISTANCE  AND  DIMENSIONS  OF  THE  SUN.  13 

Now,  in  the  case  of  the  sun  it  is  very  difficult  to 
find  the  parallax  with  sufficient  precision  on  account  of 
its  smallness — it  is  less  than  9",  almost  certainly  between 
8-75"  and  8'85".  But  this  tenth  of  a  second  of  doubtful- 
ness is  more  than  y-J-^  of  the  whole,  although  it  is  no 
more  than  the  angle  subtended  by  a  single  hair  at  a  dis- 
tance of  nearly  800  feet.  If  we  call  the  parallax  8- 80", 
which  is  probably  very  near  the  truth,  the  distance  of 
the  sun  will  come  out  92,892,000  miles,  while  a  varia- 
tion of  ^j-  of  a  second  either  way  will  change  it  about 
half  a  million  of  miles. 

When  a  surveyor  has  to  find  the  distance  of  an  in- 
accessible object,  he  lays  off  a  convenient  base-line,  and 
from  its  extremities  observes  the  directions  of  the  ob- 
ject, considering  himself  very  unfortunate  if  he  can 
not  get  a  base  whose  length  is  at  least  ^  of  the  dis- 
tance to  be  measured.  But  the  whole  diameter  of  the 
earth  is  less  than  T 1 1 0  0  of  the  distance  of  the  sun, 
and  the  astronomer  is  in  the  predicament  of  a  sur- 
veyor who,  having  to  measure  the  distance  of  an  ob- 
ject ten  miles  off,  finds  himself  restricted  to  a  base  of 
less  than  five  feet ;  and  herein  lies  the  difficulty  of  the 
problem. 

Of  course,  it  would  be  hopeless  to  attempt  this  prob- 
lem by  direct  observations,  such  as  answer  perfectly  in 
the  case  of  the .  moon,  whose  distance  is  only  thirty 
times  the  earth's  diameter.  In  her  case,  observations 
taken  from  stations  widely  separated  in  latitude,  like 
Berlin  and  the  Cape  of  Good  Hope,  or  Washington  and 
Santiago,  determine  her  parallax  and  distance  with  very 
satisfactory  precision ;  but  if  observations  of  the  same 
accuracy  could  be  made  upon  the  sun  (which  is  not  the 
case,  since  its  heat  disturbs  the  adjustments  of  an  instru- 
ment), they  would  only  show  the  parallax  to  be  some- 


14  THE  SUN. 

where  between  8*  and  10* ;  its  distance  between  126,- 
000,000  and  82,000,000  miles. 

Astronomers,  therefore,  have  been  driven  to  employ 
indirect  methods  based  on  various  principles :  some  on 
observations  of  the  nearer  planets,  some  on  calculations 
founded  upon  the  irregularities — the  so-called  pertur- 
bations— of  lunar  and  planetary  movements,  and  some 
upon  observations  of  the  velocity  of  light.  Indeed, 
before  the  Christian  era,  Aristarchus  of  Samos  had  de- 
vised a  method  so  ingenious  and  pretty  in  theory  that 
it  really  deserved  success,  and  would  have  attained  it 
were  the  necessary  observations  susceptible  of  sufficient 
accuracy. 

His  idea  was  to  observe  carefully  the  number  of 
hours  between  new  moon  and  the  first  quarter,  and  also 
between  the  quarter  and  the  full.  The  first  interval 
should  be  shorter  than  the  second,  and  the  difference 
would  determine  how  many  times  the  distance  of  the 
sun  from  the  earth  exceeds  that  of  the  moon,  as  will 
be  clear  from  the  accompanying  figure.  The  moon 

FIG.  1. 


reaches  its  quarter,  or  appears  as  a  half-moon,  when  it 
arrives  at  the  point  Q,  where  the  lines  drawn  from  it 
to  the  sun  and  earth  are  perpendicular  to  each  other. 
Since  the  angle  H  E  Q  =  E  S  Q,  it  will  follow  that 
H  Q  is  the  same  fraction  of  H  E  as  Q  E  is  of  E  S ;  so 
that,  if  H  Q  can  be  found,  we  shall  at  once  have  the 
ratio  of  Q  E  and  E  S.  Aristarchus  thought  he  had  as- 
certained that  the  first  quarter  of  the  month  (from  "N  to 


DISTANCE  AND  DIMENSIONS  OF  THE  SUN.  15 

Q)  was  about  12  hours  shorter  than  the  second,  from 
which  he  computed  the  sun  to  be  about  19  times  as  dis- 
tant as  the  moon.  The  difficulty  lies  mainly  in  the 
impossibility  of  determining  the  instant  when  the  disk 
of  the  moon  is  exactly  bisected,  and  depends  partly 
upon  the  fact  that  the  lunar  surface  is  very  rough,  and 
partly  upon  the  fact  that  the  sun's  diameter  is  nearly 
twice  that  of  the  orbit  of  the  moon,  instead  of  being  a 
mere  point,  as  in  the  figure.  The  boundary  between 
light  and  darkness — the  terminator,  as  it  is  called — is 
both  irregular  and  ill-defined.  The  real  difference  be- 
tween the  two  quarters  is  not  quite  36  minutes,  so  that 
the  sun's  distance  is  about  400  times  the  moon's.  For 
more  than  1,500  years,  however,  the  result  of  Aristar- 
chus  stood  unquestioned,  having  been  accepted  by  Hip- 
parchus  and  Ptolemy. 

The  different  methods  upon  which  our  present 
knowledge  of  the  sun's  distance  depends  may  be  classi- 
fied as  follows : 

1.  Observations  upon  the  planet  Mars  near  opposition,  in  two  dis- 

tinct ways : 

(a)  Observations  of  the  planet's  declination  made  from  sta- 
tions widely  separated  in  latitude. 

(ft)  Observations  from  a  single  station  of  the  planet's  right 
ascension  when  near  the  eastern  and  western  horizons 
— known  as  Flamsteed's  or  Bond's  method. 

2.  Observations  of  Venus  at  or  near  inferior  conjunctions: 

(a)  Observations  of  her  distance  from  small  stars  measured 
at  stations  widely  different  in  latitude. 

(ft)  Observations  of  the  transits  of  the  planet :  1.  By  noting 
the  duration  of  the  transit  at  widely-separated  sta- 
tions ;  2.  By  noting  the  true  Greenwich  time  of  con- 
tact of  the  planet  with  the  sun's  limb ;  3.  By  measur- 
ing the  distance  of  the  planet  from  the  sun's  limb  with 
suitable  micrometric  apparatus  ;  4.  By  photographing 
the  transit,  and  subsequently  measuring  the  pictures. 


16  THE  SUN. 

3.  By  observing  the  oppositions  of  the  nearer  asteroids  in  the  same 

manner  as  those  of  Mars. 

4.  By  means  of  the  so-called  parallactic  inequality  of  the  moon. 

5.  By  means  of  the  monthly  equation  of  the  sun's  motion. 

6.  By  means  of  the  perturbations  of  the  planets,  which  furnish  us 

the  means  of  computing  the  ratios  between  the  masses  of 
the  planets  and  the  sun,  and  consequently  their  distances — 
known  as  Leverrier's  method. 

7.  By  measuring  the  velocity  of  light,  and  combining  the  result 

(a)  with  "  equation  of  light "  between  the  earth  and  sun,  or 
(5)  with  "  the  constant  of  aberration." 

Our  scope  and  limits  do  not,  of  course,  require  or 
allow  any  exhaustive  discussion  of  these  different  meth- 
ods and  their  results,  but  some  of  them  will  repay  a  few 
moments'  consideration : 

The  first  three  methods,  known  as  the  trigono- 
metrical methods,  are  all  based  upon  the  same  general 
idea,  that  of  finding  the  actual  distance  of  one  of  the 
nearer  planets  by  observing  its  displacement  in  the  sky 
as  seen  from  remote  points  on  the  earth.  The  rela- 
tive distances  of  the  planets  are  easily  found  in  sev- 
eral different  ways,*  and  are  known  with  very  great 

*  One  method  of  determining  the  relative  distances  of  a  planet  and 
the  sun  from  each  other  and  from  the  earth  is  the  following,  known  since 
the  days  of  Hipparchus :  First,  observe  the  date  when  the  planet  comes 

FIG.  2. 


M 


to  its  opposition — i.  e.,  when  sun,  earth,  and  planet  are  in  line,  as  in  the 
figure,  where  the  planet  and  earth  are  represented  by  M  and  E.  Next, 
after  a  known  number  of  days,  say  one  hundred,  when  the  planet  has  ad- 


DISTANCE  AND  DIMENSIONS  OF  THE  SUN. 


17 


accuracy — the  possible  error  hardly  reaching  the  ten- 
thousandth  in  even  the  most  unfavorable  cases.  In 
other  words,  we  are  able  to  draw  for  any  moment  an 
exceedingly  accurate  map  of  the  solar  system — the  only 
question  being  as  to  the  scale.  Of  course,  the  determi- 


FIG.  8. 


nation  of  any  line  in  the  map  will  fix  this  scale ;  and 
for  this  purpose  one  line  is  as  good  as  another,  so  that 
the  measurement  of  the  distance  from  the  earth  to  the 
planet  Mars,  for  instance,  will  settle  all  the  dimensions 
of  the  system. 

vanced  to  M'  and  the  earth  to  E',  observe  the  planet's  elongation  from 
the  sun,  i.  e.,  the  angle  M'  E'  S.  Now,  since  we  know  the  periodic  times 
of  both  the  earth  and  planet,  we  shall  know  both  the  angle  M  S  M'  moved 
over  by  the  planet  in  one  hundred  days,  and  also  E  S  E',  described  in  the 
same  time  by  the  earth.  The  difference  is  M'  S  E,  often  called  the  synodic 
angle.  We  have,  therefore,  in  the  triangle  M'  S  E',  the  angle  at  E' 
measured,  and  the  angle  M'  S  E'  known  as  stated  above,  and  hence  by 
the  ordinary  processes  of  trigonometry  we  can  find  the  relative  values 
of  its  three  sides. 
3 


18  THE  SUN. 

Fig.  3  illustrates  the  method  of  observation.  Sup- 
pose two  observers,  situated  one  near  the  north  pole 
of  the  earth,  the  other  near  the  south.  Looking  at  the 
planet,  the  northern  observer  will  see  it  at  N  (in  the  upper 
figure),  while  the  other  will  see  it  at  S,  farther  north  in 
the  sky.  If  the  northern  observer  sees  it  as  at  A  (in 
the  lower  part  of  the  figure),  the  southern  will  at  the 
same  time  see  it  as  at  B ;  and,  by  measuring  carefully 
at  each  station  the  apparent  distance  of  the  planet  from 
several  of  the  little  stars  (a,  J,  c)  which  appear  in  the 
field  of  view,  the  amount  of  the  displacement  can  be 
accurately  ascertained.  The  figure  is  drawn  to  scale. 
The  circle  E  being  taken  to  represent  the  size  of  the 
earth  as  seen  from  Mars  when  nearest  us,  the  black  disk 
represents  the  apparent  size  of  the  planet  on  the  same 
scale,  and  the  distance  between  the  points  N  and  S,  in 
either  figure  A  or  B,  represents,  on  the  same  scale  also, 
the  displacement  which  would  be  produced  in  the  plan- 
et's position  by  a  transference  of  the  observer  from 
Washington  to  Santiago,  or  vice  versa. 

The  first  modern  attempt  to  determine  the  sun's 
parallax  was  made  by  this  method  in  16TO,  when  the 
French  Academy  of  Sciences  sent  Richer  to  Cayenne  to 
observe  the  opposition  of  Mars,  while  Cassini  (who  pro- 
posed the  expedition),  Roemer,  and  Picard  observed  it 
from  different  stations  in  France.  When  the  results 
came  to  be  compared,  however,  it  was  found  that  the 
planet's  displacement  was  imperceptible  by  their  exist- 
ing means  of  observation  :  from  this  they  inferred  that 
the  planet's  parallax  could  not  exceed  half  a  minute  of 
arc,  and  that  the  sun's  could  not  be  more  than  10". 

In  1Y52  Lacaille  at  the  Cape  of  Good  Hope  made 
similar  observations,  and  their  comparison  with  cor- 
responding observations  in  Europe  showed  that  instru- 


DISTANCE  AND  DIMENSIONS  OF  THE  SUN.  19 

merits  had  so  far  improved  as  to  make  the  displacement 
quite  sensible.  He  fixed  the  sun's  parallax  at  10",  cor- 
responding to  a  distance  of  82,000,000  miles. 

In  more  recent  times  the  method  has  been  frequently 
applied.  It  can  be  used  to  the  best  advantage,  of  course, 
when,  at  the  time  of  "  opposition,"  the  planet  is  near  its 
perihelion  and  the  earth  near  its  aphelion,  for  then  the 
distance  between  Mars  and  the  earth  is  the  least  possi- 
ble. These  favorable  oppositions  occur  in  the  late  sum- 
mer or  early  autumn  about  once  in  fifteen  years,  as  in 
1847,  1862,  1877,  and  1892. 

The  meridian  observations  which  furnish  the  ma- 
terial of  method  l<z,  and  were  mainly  relied  on  until  re- 
cently, seem  for  some  reason,  perhaps  connected  with 
the  planet's  red  color,  to  be  untrustworthy  ;  at  any  rate, 
they  generally  give  values  of  the  parallax  persistently 
about  one  per  cent,  larger  than  the  other  methods,  and 
rather  discordant  among  themselves. 

Flamsteed's  method,  on  the  other  hand,  stands  very 
high,  especially  when  so  modified  as  to  utilize  the  co- 
operation of  numerous  observers  in  different  countries. 
Though  first  suggested  long  ago,  it  would  have  amounted 
to  very  little  with  the  instruments  then  available,  and  it 
had  been  practically  lost  sight  of  until  the  expedition  of 
Dr.  Gill  to  Ascension  Island,  in  1877,  brought  out  its 
real  value. 

His  instrument  was  a  "  heliometer,"  loaned  by  Lord 
Lindsay  for  the  occasion.  It  consists  essentially  of  a 
telescope  having  its  object-glass  divided  into  two  semi- 
circular pieces  which  can  slide  by  each  other.  Each 
half  of  the  lens  makes  its  own  image  of  the  object  under 
examination,  so  that  by  properly  setting  the  semi-lenses 
the  images  of  two  neighboring  stars  can  be  made  to 
coincide ;  and  if  we  know  the  displacement  of  the  two 


20  THE  SUN. 

lenses,  which  can  be  measured  by  an  accurate  scale,  the 
angular  distance  between  the  stars  can  be  determined 
with  a  precision  unattainable  by  any  other  known  pro- 
cess.  The  instrument  is  delicate,  complicated,  and  diffi- 
cult to  use,  but  in  the  hands  of  an  adept  it  is  thoroughly 
reliable.  It  was  with  the  heliometer  that  Bessel,  in  1838, 
first  sounded  interstellar  space  by  measuring  the  annual 
parallax*  and  distance  of  61  Cygni. 

Mr.  Gill's  observations  consisted  in  measurements 
of  the  apparent  distance  between  the  planet  and  the 
stars  lying  near  its  path,  and  of  the  distances  between 
the  stars  themselves ;  the  principal  observatories  also 
co-operated  in  the  work  by  determining  with  the  utmost 
precision  the  absolute  places  of  the  stars.  It  would 
take  too  much  space  to  explain  fully  how  from  such 
observations  the  solar  parallax  can  be  accurately  worked 
out ;  but  any  one  can  easily  see  that  when  the  planet  is 
rising  the  effect  of  parallax  (which  always  makes  a  body 
appear  lower  in  the  heavens  than  it  otherwise  would)  is  to 
shift  it  apparently  toward  the  east;  when  Mars  is  in  the 
west  the  apparent  shift,  on  the  other  hand,  is  westward  ; 
and  by  comparing  the  measurements  made  at  all  hours 
of  the  night  for  several  consecutive  weeks  the  planet's 
regular  orbital  motion  and  the  amount  of  this  daily 
parallactic  shift  can  be  separately  determined  with  mi- 
nute exactness. 

As  the  final  result  of  the  whole  operation,  Dr.  Gill 
obtained  8'780"  ±  0'020"  for  the  sun's  parallax. 

Several  of  the  minor  planets  or  asteroids  which  have 
very  eccentric  orbits  at  times  come  so  near  us  at  oppo- 

*  The  "  annual,"  or  "  heliocentric,"  "  parallax  "  of  a  star  is  not  the 
same  as  its  horizontal  parallax,  or  angular  semi-diameter  of  the  earth  as 
seen  from  the  star;  it  is  the  semi-diameter  of  the  earth's  orbit  viewed 
from  the  star,  and  is  nearly  twelve  thousand  times  greater  than  the  other. 


DISTANCE   AND   DIMENSIONS   OF   THE   SUN.  21 

sition  that  they  can  be  advantageously  observed  in  the 
same  way.  They  never  approach  quite  as  close  as  Mars 
does,  but  per  contra  they  are  so  much  smaller  that 
they  look  just  like  stars,  and  can  be  observed  with  the 
heliometer  much  more  accurately  than  a  planet  which 
presents  a  disk.  Very  recently,  in  1889  and  1890,  a 
concerted  system  of  observations  was  made  upon  Vic- 
toria, Iris,  and  Sappho  by  Dr.  Gill,  now  the  Astronomer 
Royal  at  the  Cape  of  Good  Hope ;  Dr.  Elkin,  of  the 
Yale  College  Observatory  (which  possesses  a  fine  heli- 
ometer, an  exact  mate  of  Dr.  Gill's,  and  the  only  one 
in  the  United  States),  and  two  or  three  German  ob- 
servers with  smaller  instruments.  The  results  are  very 
satisfactory,  ranging  from  8'796"  to  8*825",  the  mean 
being  8-807",  with  a  probable  error  of  only  0'006". 

So  far  as  can  be  judged  from  the  details  thus  far 
published,  this  determination  must  be  conceded  the  pre- 
cedence over  all  others  in  respect  to  its  probable  freedom 
from  constant  and  systematic  errors,  and  from  theoret- 
ical difficulties. 

In  observations  of  this  sort  upon  Mars  or  the  aster- 
oids, the  position  and  displacement  of  the  planet,  as 
seen  from  different  stations,  are  determined  by  com- 
paring it  with  neighboring  stars.  When  Yenus,  how- 
ever, is  nearest  us,  she  can  be  observed  only  by  day,  so 
that  in  her"  case  star  comparisons  are  as  a  general  thing 
out  of  the  question.  But  occasionally  at  her  inferior 
conjunction  she  passes  directly  across  the  disk  of  the 
sun,  the  phenomenon  being  known  as  a  "transit" 
These  transits  are  very  rare,  coming  (at* present)  in, 
pairs,  the  two  transits  which  constitute  a  pair  being 
separated  by  an  interval  of  eight  years,  while  between 
the  pairs  themselves  there  is  an  interval  of  either  one 
hundred  and  thirty  or  one  hundred  and  thirteen  years. 


22  THE  SUN. 

They  occur  either  in  June  or  December,  and  thus  far 
there  have  been  six  since  the  invention  of  the  telescope ; 
viz.,  in  December,  1631  and  1639 ;  in  June,  1761  and 
1769 ;  and  in  December,  1874  and  1882,  the  next  pair 
being  due  in  June,  2004  and  2012. 

On  these  occasions  the  parallactic  displacement  oi 
the  planet  as  seen  from  different  stations  can  be  deter- 
mined by  making  any  such  observations  as  will  enable 
the  computer  to  ascertain  accurately  her  apparent  dis- 
tance and  direction  from  the  sun's  center  at  some  given 
moment. 

Gregory  in  1663  first  pointed  out  the  utility  of  such 
observations  for  ascertaining  the  parallax,  but  it  was 
not  until  some  fifteen  years  later  that  the  subject  was 
fairly  brought  to  the  attention  of  astronomers  by  Hal- 
ley,  who  discussed  the  matter  thoroughly,  and  showed 
how  the  problem  might  be  solved  with  accuracy  by 
observations  such  as  were  practicable  even  with  the 
instruments  and  knowledge  then  at  command. 

From  that  time  for  fully  two  hundred  years  it  was 
the  almost  universal  opinion  of  astronomers  that  no 
other  method  could  rival  this  as  a  means  of  determining 
the  distance  of  the  sun. 

The  transits  of  1761  and  1769  were  observed  in  all 
accessible  quarters  of  the  globe  by  expeditions  sent  out 
by  the  different  governments.  From  different  sets  of 
these  observations  variously  combined  by  different  com- 
puters, values  of  the  solar  parallax  were  obtained  ranging 
all  the  way  from  7'5"  to  9-2".  A  general  discussion  of 
all  the  material  afforded  by  the  two  transits  was  first 
made  by  Encke  in  1822,  and  he  obtained,  as  the  most 
probable  result,  the  value  8*5776",  which  from  that  time 
for  more  than  thirty  years  was  accepted  by  all  astrono- 
mers as  the  best  attainable  approximation  to  the  truth. 


DISTANCE  AND   DIMENSIONS  OF  THE  SUN. 


23 


In  1854  Hansen,  in  publishing  some  of  his  results 
respecting  the  motion  of  the  moon,  announced  that 
Encke's  value  of  the  solar  parallax  could  not  be  recon- 
ciled with  his  investigations ;  within  the  next  six  or  seven 
years  several  independent  researches  by  other  astrono- 
mers confirmed  his  conclusions ;  and  the  more  recent 
recomputations  by  Powalky,  Stone,  Faye,  and  others, 
show  that  the  errors  of  observation  were  so  considerable 
in  1769  that  nothing  more  can  be  fairly  deduced  from 
that  transit  than  that  the  solar  parallax  is  probably 
somewhere  between  8 "7"  and  8 '9". 

The  method  of  observation  then  used  consisted  sim- 
ply in  noting  the  moment  when  the  limb  of  the  planet 
came  in  contact  with  that  of  the  sun — an  observation 
which  is  attended  with  much  more  difficulty  and  uncer- 
tainty than  would  at  first  be  supposed.  The  difficul- 
ties depend  in  part  upon  the  imperfections  of  optical 
instruments  and  the  human  eye,  partly  upon  the  essen- 


tial  nature  of  light,  leading  to  what  is  known  as  diffrac- 
tion, and  partly  upon  the  action  of  the  planet's  atmos- 
phere. The  two  first-named  causes  produce  what  is 
called  irradiation,  and  operate  to  make  the  apparent 
diameter  of  the  planet,  as  seen  on  the  solar  disk,  small- 


24  THE   SUN. 

er  than  it  really  is — smaller,  too,  by  an  amount  which 
varies  with  the  size  of  the  telescope,  the  perfection  of 
its  lenses,  and  the  tint  and  brightness  of  the  sun's  image. 
The  edge  of  the  planet's  image  is  also  rendered  slightly 
hazy  and  indistinct. 

The  planet's  atmosphere  also  causes  its  disk  to  be 
surrounded  by  a  narrow  ring  of  light,  which  becomes 
visible  long  before  the  planet  touches  the  sun,  and  at 
the  moment  of  internal  contact  produces  an  appearance 
of  which  the  accompanying  figure  is  intended  to  give 
an  idea,  though  on  an  exaggerated  scale.  The  planet 
moves  so  slowly  as  to  occupy  more  than  twenty  minutes 
in  crossing  the  sun's  limb ;  so  that,  even  if  the  planet's 
edge  were  perfectly  sharp  and  definite,  and  the  sun's 
limb  undistorted,  it  would  be  very  difficult  to  deter- 
mine the  precise  second  at  which  contact  occurs ;  but  as 
things  are,  observers  with  precisely  similar  telescopes, 
and  side  by  side,  often  differ  from  each  other  five  or 
six  seconds ;  and  where  the  telescopes  are  not  similar 
the  differences  and  uncertainties  are  much  greater. 
The  extent  of  the  difficulty  can  be  judged  of  by  the 
simple  fact  that,  from  the  whole  mass  of  contact  obser- 
vations obtained  in  1874  by  the  different  British  parties 
which  observed  the  transit,  three  different  values  of  the 
solar  parallax  have  been  deduced  by  different  computers, 
viz.,  the  official  value  8-76"  by  Airy,  8'81"  by  Tupman, 
and  8*88*  by  Stone.  These  differences  depend  mainly 
upon  the  different  interpretations  given  to  the  de- 
scription of  phenomena  noted  by  the  observers  in  the 
field. 

In  1882  things  were  perhaps  a  little  better,  as  many 
of  the  observers  had  the  benefit  of  experience  in  1874. 
Professor  ISTewcomb  deduces  from  all  the  observations 
of  internal  contact  in  the  two  transits  a  solar  parallax 


DISTANCE  AND   DIMENSIONS  OF  THE  SUN.  25 

of  8-TW  ±  0-023".  But  many  of  the  several  hundred 
observations  were  seriously  discordant. 

The  difficulties  of  the  case  were  fully  realized  at  the 
time  when  preparations  were  making  for  the  observa- 
tion of  the  transit  of  1874,  and  astronomers  were  dis- 
posed to  put  more  reliance  upon  micrometric  and  pho- 
tographic methods,  which  are  free  from  these  peculiar 
difficulties,  though  of  course  beset  with  others,  which, 
however,  it  was  hoped  would  prove  less  formidable. 

All  the  numerous  expeditions,  therefore,  which  were 
sent  out  by  the  various  governments  to  observe  the 
transits  of  18Y4  and  1882  were  equipped  to  use  one  or 
both  these  methods. 

All  the  eight  German  parties,  two  or  three  of  the 
Russian  parties,  one  English,  and  one  Belgian  party 
were  provided  with  heliometers,  and  busied  themselves 
during  the  transit  with  measuring  the  distance  of  the 
planet  from  the  edge  of  the  sun's  disk.  The  results  of 
the  German  observations  have  been  fully  worked  out 
and  published.  From  the  four  hundred  and  forty-six 
different  measures  Auwers  deduces  a  solar  parallax  of 
8-8  W  ±  0'04:0" ;  the  value  is  surprisingly  large,  but  the 
magnitude  of  its  probable  error  indicates  that  the  obser- 
vations did  not  agree  very  closely. 

The  Americans  and  French  placed  their  main  reli- 
ance upon  the  photographic  method,  while  the  English 
and  Germans  also  provided  for  its  use  to  a  certain 
extent.  The  great  advantage  of  this  method  is  that 
it  makes  it  possible  to  perform  the  necessary  measure- 
ments, upon  whose  accuracy  everything  depends,  at 
leisure  after  the  transit,  without  hurry,  and  with  all 
possible  precautions.  The  field-work  consists  merely  in 
obtaining  as  many  and  as  good  pictures  as  possible.  A 
principal  objection  to  the  method  lies  in  the  difficulty 


26  THE  SUN. 

of  obtaining  good  pictures — i.  e.,  pictures  free  from  dis- 
tortion, and  so  distinct  and  sharp  as  to  bear  high  mag- 
nifying power  in  the  microscopic  apparatus  used  for 
their  measurement.  A  most  serious  difficulty,  more- 
over, is  involved  in  the  accurate  determination  of  the 
scale  of  the  picture — that  is,  of  the  number  of  seconds 
of  arc  corresponding  to  a  linear  inch  upon  the  plate. 
Besides  this,  we  must  know  the  exact  Greenwich  time 
at  which  each  picture  is  taken,  and  it  is  also  extremely 
desirable  that  the  orientation  of  the  picture  should  be 
accurately  determined — that  is,  the  north  and  south,  east 
and  west  points  of  the  solar  image  on  the  finished  plate. 
There  has  been  a  good  deal  of  anxiety  lest  the  image, 
however  accurate  and  sharp  when  first  produced,  should 
alter  in  course  of  time  through  the  contraction  of  the 
collodion  or  gelatine  film  on  the  glass  plate,  but  the  ex- 
periments of  Rutherfurd,  Huggins,  and  Paschen  seem 
to  show  that  this  danger  is  imaginary. 

The  uncertainty  of  our  present  knowledge  of  the 
sun's  parallax  is,  however,  so  small  that  we  can  hope  to 
improve  it  only  by  means  of  photographs  that  are 
almost  absolutely  perfect.  Unless  the  picture  is  so  dis- 
tinct and  free  from  distortion  that  the  relative  positions 
of  Venus  and  the  sun's  center  can  be  determined  from 
it  on  the  four-inch  disk  within  3^  of  an  inch  the  plate 
is  practically  worthless. 

But  it  is  to  be  noted  that  any  mere  enlargement  or 
diminution  of  the  diameter  of  sun  or  planet  will  do  no 
harm,  provided  it  is  alike  all  around  the  circumference 
of  the  disk,  since  the  measurement  is  not  from  the  edge 
of  Venus  to  the  edge  of  the  sun,  but  between  their  cen- 
ters. Photographic  determinations  of  contact,  on  the 
contrary  (such  as  Janssen  and  some  of  the  English  par- 
ties attempted  by  a  peculiar  and  complicated  apparatus), 


DISTANCE  AND   DIMENSIONS  OF  THE  SUN.  27 

are  affected  with  all  the  uncertainties  of  the  old-fash- 
ioned observations  of  the  eye  alone,  and  with  others  in 
addition ;  so  that,  astronomically  considered,  they  are 
entirely  worthless,  although  interesting  from  a  chemical 
and  physical  point  of  view. 

In  1874  two  essentially  different  lines  of  proceeding 
were  adopted  in  the  photographic  observations.  The 
English  and  Germans  attached  a  camera  to  the  eye- 
end  of  an  ordinary  telescope,  which  was  pointed  directly 
at  the  sun  ;  the  image  formed  at  the  focus  of  the  tele- 
scope was  enlarged  to  the  proper  size  by  a  combination 
of  lenses  in  the  camera ;  and  a  small  plate  of  glass  ruled 
with  squares  was  placed  at  the  focus  of  the  telescope 
and  photographed  with  the  sun's  image,  furnishing  a 
set  of  reference-lines,  which  give  the  means  of  detecting 
and  allowing  for  any  distortion  caused  by  the  enlarging 
lenses. 

The  Americans  and  French,  on  the  other  hand,  pre- 
ferred to  make  the  picture  of  full  size,  without  the  in- 
tervention of  any  enlarging  lens :  as  this  requires  an 
object-glass  with  a  focal  length  of  thirty  or  forty  feet, 
which  could  not  be  easily  pointed  at  the  sun,  a  plan 
proposed  first  by  M.  Laussedat,  but  also  independently 
by  our  own  Professor  Winlock,  was  adopted.  The  tele- 
scope is  placed  horizontal,  and  the  rays  are  reflected  into 
the  object-glass  by  a  plane  mirror  suitably  mounted. 
The  French  used  mirrors  of  silvered  glass,  and  took 
their  pictures  (about  two  and  a  half  inches  in  diame- 
ter) by  the  old  daguerreotype  process  on  silvered  plates 
of  copper,  in  order  to  avoid  the  risk  of  collodion-con- 
traction. With  the  silvered  mirror  the  time  of  expo- 
sure is  so  short  that  no  clock-work  is  required.  The 
Americans  used  unsilvered  mirrors,  to  obviate  any  dis- 
torting action  of  the  sun's  rays  upon  the  form  of  the 


28 


THE  SUN. 


mirror.  This,  of  course,  made  the  light  feebler  and 
the  time  of  exposure  longer,  so  that  a  clock-work  move- 
ment of  the  mirror  was  needed  to  keep  the  image  from 


FIG.  5. 


AMERICAN  APPARATUS  FOE  PHOTOGRAPHING  THE  TRANSIT  OF  VENUS. 

changing  its  place  on  the  plate  during  the  exposure, 
which,  however,  never  exceeded  half  a  second.  Fig. 
5,  taken  from  the  author's  "  General  Astronomy "  by 
permission  of  the  publishers,  gives  an  idea  of  the  ar- 
rangement. The  pier  that  carries  the  plate  was  in  a 
darkened  room,  into  which  the  rays  from  the  mirror 
were  admitted  by  a  sliding  shutter. 

In  1874  the  American  pictures  were  taken  by  the 
ordinary  wet  process  on  glass,  and  were  about  four 
inches  in  diameter.  In  1882  a  gelatine  emulsion  pro- 
cess was  used.  Just  in  front  of  the  sensitive  plate,  at  a 
distance  of  about  one  eighth  of  an  inch,  was  placed  a 
reticle,  or  a  plate  of  glass  ruled  in  squares,  and  between 
this  and  the  collodion-plate  hung  a  fine  silver  wire  sus- 
pending a  plumb-bob.  Thus  the  finished  negative  was 


DISTANCE  AND  DIMENSIONS  OF  THE  SUN.  29 

marked  into  squares,  and  also  bore  the  image  of  the 
plumb-line,  which  indicated  precisely  the  direction  of 
the  vertical.  The  Americans  also  placed  the  photo- 
graphic telescope  exactly  in  line  with  a  meridian  instru- 
ment, and  so  determined,  with  the  extremest  precision, 
the  direction  in  which  it  was  pointed.  Knowing  this, 
and  the  time  at  which  any  picture  was  taken,  it  becomes 
possible,  with  the  help  of  the  plumb-line  image,  to  de- 
termine precisely  the  orientation  of  the  picture — an  ad- 

Fio.  6. 


vantage  possessed  by  the  American  pictures  alone,  and 
making  their  value  nearly  twice  as  great  as  otherwise  it 
would  have  been. 

The  above  figure  is  a  representation  of  one  of  the 
American  photographs  reduced  about  one  half.  Vis 
the  image  of  Yenus,  which  on  the  actual  plate  is  about 
one  seventh  of  an  inch  in  diameter ;  a  a'  is  the  image 
of  the  plumb-line.  The  center  of  the  reticle  is  marked 


30  THE  SUN. 

by  the  little  cross,  and  the  word  "  China,"  written  on 
the  reticle-plate  with  a  diamond — and,  of  course,  copied 
on  the  photograph — indicates  that  it  is  one  of  the  Peking 
pictures.  Its  number  in  the  series  is  given  in  the  right- 
hand  upper  corner.  About  90  such  pictures  were  ob- 
tained at  Peking  during  the  transit,  and  about  350  at 
all  the  eight  American  stations,  the  work  being  much 
interfered  with  by  unfavorable  weather  at  most  of  them. 
If  we  add  those  obtained  by  the  French,  Germans,  and 
English,  the  total  number  available  reaches  nearly  1,200, 
according  to  the  best  estimates. 

After  the  pictures  are  made  and  safely  brought 
home,  they  have  next  to  be  measured — i.  e.,  the  dis- 
tance (and  in  the  American  pictures  the  direction  also) 
between  the  center  of  Venus  and  the  center  of  the  sun 
must  be  determined  in  each  picture.  This  is  an  exceed- 
ingly delicate  and  tedious  operation,  rendered  more  dif- 
ficult by  the  fact  that  the  image  of  the  sun  is  never 
truly  circular,  but,  even  supposing  the  instrument  to  be 
perfect  in  all  its  adjustments,  is  somewhat  distorted  by 
the  effect  of  atmospheric  refraction ;  so  that  the  true 
position  of  the  sun's  center  with  reference  to  the  squares 
of  the  reticle  is  determined  only  by  an  intricate  calcula- 
tion from  measurements  made  with  a  microscopic  ap- 
paratus on  a  great  number  of  points  suitably  chosen  on 
the  circumference  of  the  image.  The  final  result  of  the 
measurement  comes  out  something  in  this  form :  Peking. 
No.  32.  Time,  14h  08m  20'28  (Greenwich  mean  time) ; 
Yenus  north  of  sun's  center,  735-32" ;  east  of  center, 
441-63";  distance  from  center  of  sun,  85T75".  (The 
numbers  given  are  only  imaginary.) 

In  1882  less  prominence  was  given  to  photographic 
operations  by  most  of  the  Government  expeditions,  since 
the  results  of  the  work  in  1874:,  so  far  as  then  published, 


DISTANCE  AND   DIMENSIONS  OF  THE   SUN.  31 

were  not  very  satisfactory.  The  American  parties, 
however,  adhered  to  the  same  apparatus  and  methods  as 
in  1874,  except  that  the  collodion  process  was  replaced 
by  an  emulsion.  Nearly  1,500  photographs  were  ob- 
tained. From  the  whole  system  of  American  photo- 
graphs Professor  JSTewcomb  deduces  a  solar  parallax  of 
8-857"  ±  0-016".  The  measures  of  distance  alone  give 
8*867",  but  those  of  position-angles  gave  8-873"  in  1874, 
and  8-772"  in  1882. 

The  discordances  between  the  results  from  different 
plates,  made  within  a  few  minutes  of  each  other,  show 
that  there  is  something  wrong  with  the  method.  The 
most  probable  explanation  is  perhaps  to  be  found  in  the 
distortions  suffered  by  the  plane  mirror  of  the  apparatus 
under  changes  of  position  and  temperature. 

From  the  92  French  daguerreotypes  of  1874  a  paral- 
lax of  8-80"  ±  0-03"  was  deduced  by  Obrecht. 

The  English  photographs  of  1874  proved  of  little 
value.  They  were  measured  by  two  different  persons, 
and  from  the  measurements  of  one  (Mr.  Burton)  a  par- 
allax of  8  "25"  was  deduced,  while  from  those  of  the 
other  (Captain  Tupman)  the  result  was  8*08*.  One  of 
the  principal  difficulties  evidently  lay  in  the  uncertainty 
of  the  scale-value,  which  was  only  deduced  from  the  di- 
ameters of  the  sun  and  planet. 

On  the  whole,  it  may  be  taken  as  certain  that  here- 
after transits  of  Yenus  will  not  be  considered  of  such 
supreme  importance  as  in  the  past.  Other  less  costly 
operations  will  give  better  results  for  the  solar  parallax. 

The  methods  numbered  4,  5,  and  6,  on  page  16, 
are  usually  classed  together  as  "gravitational"  since 
they  depend  on  calculations  which  are  founded  on  the 
law  of  gravitation.  One  of  the  best  of  them  is  based 
upon  the  careful  observation  of  the  motions  of  the 


32  THE  SUN. 

moon.  The  first  suspicion  as  to  the  correctness  of  the 
then  received  distance  of  the  sun  was  raised  in  1854  by 
Hansen's  announcement  that  the  moon's  parallactic  in- 
equality led  to  a  smaller  value  than  that  deduced  from 
the  transit  of  Yenus — a  conclusion  corroborated  by 
Leverrier  four  jears  later,  from  the  so-called  lunar 
equation  of  the  sun's  motion.  It  seems  at  first  sight 
strange,  but  it  is  true,  as  Laplace  long  since  pointed  out, 
that  the  skillful  astronomer,  by  merely  watching  the 
movements  of  our  satellite,  and  without  leaving  his  ob- 
servatory, can  obtain  the  solution  of  problems  which, 
attacked  by  other  methods,  require  tedious  and  expen- 
sive expeditions  to  remote  corners  of  the  earth.  Our 
scope  and  object  do  not  require  us  to  enter  into  detail 
respecting  this  lunar  method  of  finding  the  sun's  paral- 
lax ;  it  must  suffice  to  say  that  the  disturbing  action  of 
the  sun  makes  the  interval  from  new  moon  to  the  first 
quarter  about  eight  minutes  longer  than  that  from  the 
quarter  to  full ;  and  this  difference  depends  upon  the 
ratio  between  the  diameter  of  the  moon's  orbit  and  the 
distance  of  the  sun  in  such  a  manner  that,  if  the  in- 
equality is  accurately  observed,  the  ratio  can  be  cal- 
culated. Since  we  know  the  distance  of  the  moon,  this 
will  give  that  of  the  sun.  The  results  obtained  in  this 
way,  according  to  the  most  recent  investigations,  ap- 
pear to  fix  the  solar  parallax  between  8'767"  and  8'802". 
Newcomb  assigns  S'T^"  as  the  weighted  mean. 

But  the  method  by  which  ultimately  we  shall  obtain 
the  most  accurate  determination  of  the  dimensions  of 
our  system  is  that  proposed  by  Leverrier,  depending 
upon  the  secular  perturbations  produced  by  the  earth 
upon  her  neighboring  planets ;  especially  in  causing  the 
motions  of  their  nodes  and  perihelia.  These  motions 
are  very  slow,  but  continuous  ;  and  hence,  as  time  goes 


DISTANCE  AND  DIMENSIONS  OF  THE  SUN.  33 

on,  they  will  become  known  with  ever-increasing  accu- 
racy. If  they  were  known  with  absolute  precision,  they 
would  enable  us  to  compute,  with  absolute  precision 
also,  the  ratio  between  the  masses  of  the  sun  and  earth, 
and  from  this  ratio  we  can  calculate  *  the  distance  of 
the  sun  by  either  of  two  or  three  different  methods. 

As  matters  stand  at  present,  the  majority  of  astrono- 
mers would  probably  consider  that  these  secular  pertur- 
bations are  not  yet  known  with  an  exactness  sufficient 
to  render  this  method  superior  to  the  others  that  have 
been  named — perhaps  as  yet  not  even  their  rival.  Le- 
verrier,  on  the  other  hand,  himself  put  such  confidence 
in  it  that  he  declined  to  sanction  or  co-operate  in  the 
operations  for  observing  the  recent  transit  of  Yenus, 
considering  all  labor  and  expense  in  that  direction  as 
merely  so  much  waste. 

But,  however  the  case  may  be  now,  there  is  no 
question  that  as  time  goes  on,  and  our  knowledge  of 
the  planetary  motions  becomes  more  minutely  precise, 
this  method  will  become  continually  and  cumulatively 
more  exact,  until  finally,  and  not  many  centuries  hence, 
it  will  supersede  all  the  others  that  have  been  described. 

*  One  method  of  proceeding  is  as  follows :  Let  M  be  the  mass  of  the 
sun  and  earth  united,  and  m  that  of  the  earth  and  moon ;  let  R  be  the 
distance  of  the  sun  from  the  earth,  and  r  that  of  the  moon ;  finally,  let 
T  be  the  number  of  days  in  a  sidereal  year,  and  t  the  number  in  a  side- 
real month.  Then,  by  elementary  astronomy — 


M:m  =  ^:n:  whence  R*  =  r«   4r         ^ 


Hfi  00- 


or,  in  words,  the  cube  of  the  sun's  distance  equals  the  cube  of  the  moon's 
distance,  multiplied  by  the  square  of  the  number  of  sidereal  months  in  a 
year,  and  by  the  ratio  between  the  masses  of  the  swr>  and  earth.  It  is  to  be 
noted,  however,  that  T  and  /  are  the  periods  of  die  earth  and  moon,  as 
they  would  be  if  wholly  undisturbed  in  their  motions,  and  hence  differ 
slightly  from  the  periods  actually  observed — the  differences  are  small, 
but  somewhat  troublesome  to  calculate  with  precision. 
4 


34  THE  SUN. 

The  parallax  of  the  sun  determined  by  Leverrier  in 
this  method,  in  1872,  came  out  8'86". 

Professor  Newcomb,  as  the  result  of  his  recent  ex- 
haustive researches  upon  the  subject,  gets  8'759"  ± 
0-010". 

The  last  of  the  methods  mentioned  in  the  synopsis 
given  on  pages  15  and  16  is  interesting  as  an  example  of 
the  manner  in  which  the  sciences  are  mutually  connected 
and  dependent.  Before  the  experiments  of  Fizeau  in 
1849,  and  of  Foucault  a  few  years  later,  our  knowledge 
of  the  velocity  of  light  depended  on  our  knowledge  of 
the  dimensions  of  the  earth's  orbit.  It  had  been  found 
by  astronomical  observations  upon  the  eclipses  of  Jupi- 
ter's satellites  that  light  occupied  a  little  more  than  six- 
teen minutes  in  crossing  the  orbit  of  the  earth,  or  about 
eight  minutes  in  coming  from  the  sun;  and  hence, 
supposing  the  sun's  distance  to  be  95,600,000  miles,  as 
was  long  believed,  the  velocity  of  light  must  be  about 
192,000  miles  per  second.  Thus  optics  was  indebted  to 
astronomy  for  this  fundamental  element.  But  when 
Foucault  in  1862  announced  that,  according  to  his  un- 
questionably accurate  experiments,  the  velocity  of  light 
could  not  be  much  more  than  186,000  miles  per  second, 
the  obligation  was  returned,  and  the  suspicions  as  to 
the  received  value  of  the  sun's  parallax,  which  had  been 
raised  by  the  lunar  researches  of  Hanseri  and  Leverrier, 
were  changed  into  certainty. 

The  most  accurate  determinations  of  the  velocity  of 
light  have  been  made  in  this  country  by  Michelson  and 
Newcomb,  between  1879  and  1883,  and  give  as  the  re- 
sult 186,327  miles,  with  a  probable  error  not  exceeding 
twenty  miles. 

From  this  we  can  derive  the  distance  of  the  sun  di- 
rectly by  merely  multiplying  it  by  the  "  constant  of  the 


DISTANCE  AND   DIMENSIONS  OF  THE  SUN.  35 

equation  of  light"  which  is  simply  the  number  of  seconds 
required  by  light  to  travel  from  the  sun  to  the  earth. 
This  "constant"  is  determined  by  observation  upon  the 
eclipses  of  Jupiter's  satellites,  and  is  almost  certainly 
very  near  499  seconds,  though  still  doubtful  by  a  frac- 
tion of  a  second.  This  would  give  92,977,000  miles  for 
the  sun's  distance,  corresponding  to  a  parallax  of  about 
8*79".  During  the  last  twelve  or  fifteen  years  continu- 
ous series  of  observations  have  been  in  progress  by  new 
photometric  methods  both  at  Cambridge  (U.  S.)  and 
Paris,  and  when  their  result  is  published  we  shall  un- 
doubtedly have  a  much  more  accurate  value  of  the  light- 
equation. 

The  velocity  of  light  may  be  utilized  in  another  way 
to  solve  the  problem,  by  combining  it  with  the  so-called 
"  constant  of  aberration."  This  "  aberration  constant" 
is  deduced  from  observations  upon  the  fixed  stars,  and 
almost  certainly  lies  somewhere  between  20*4:5"  and 
20-55",  corresponding  to  parallaxes  of  8'81"  and  8'77". 
Its  determination,  however,  is  somewhat  embarrassed  by 
the  newly  discovered  "  variation  of  latitude,"  and  it  is 
expected  that  new  determinations,  in  which  this  varia- 
tion is  duly  eliminated  or  taken  into  account,  will  give 
a  much  more  accurate  value  of  the  aberration. 

The  only  difficulty  wtth  these  two  methods  lies  in 
the  theoretical  question  whether  we  can  safely  assume 
that  in  interplanetary  space  the  velpcity  of  light  is 
identical  with  that  determined  by  experiments  made  at 
the  surface  of  the  earth,  even  after  all  known  corrections 
for  the  density  of  the  air,  etc.,  have  been  applied. 

Admitting  it,  there  can  hardly  be  a  doubt  that  this 
"physical  method"  as  it  is  often  called,  outranks  all 
others  for  the  present  as  a  means  of  determining  the 
distance  of  the  sun ;  and  the  reader's  attention  is  called 


36  THE  SUN. 

to  the  fact  that  it  gives  directly  the  distance  of  the  sun, 
and  the  parallax  only  indirectly.  It  does  not  depend 
at  all  upon  our  measures  of  the  dimensions  or  gravita- 
tional attraction  of  the  earth. 

Collecting  all  the  evidence  at  present  attainable,  it 
would  seem  that  the  solar  parallax  can  not  differ  much 
from  8'SO*,  though  it  may  be  as  much  as  O'Ol"  greater  or 
smaller ;  this  would  correspond,  as  has  already  been  said, 
to  a  distance  of  92,892,000  miles,  with  a  probable  error 
of  about  one  eighth  of  one  per  cent.,  or  120,000  miles.* 

But,  though  the  distance  can  easily  be  stated  in  fig- 
ures, it  is  not  possible  to  give  any  real  idea  of  a  space 
so  enormous ;  it  is  quite  beyond  our  power  of  concep- 
tion. If  one  were  to  try  to  walk  such  a  distance,  sup- 
posing that  he  could  walk  4  miles  an  hour,  and  keep  it 
up  for  10  hours  every  day,  it  would  take  68-J  years  to 

*  The  oscillations  of  scientific  opinion  as  to  the  value  of  this  constant 
have  been  very  curious.  Early  in  the  century  Laplace,  in  the  "  Mecanique 
Celeste,"  adopted  the  value  8-81"  given  by  the  first  discussion  of  the  tran- 
sits of  Venus  in  1761-'69  ;  but  other  astronomers,  Delambre,  for  instance, 
proposed  a  smaller  value.  Encke,  as  has  been  said  before,  made  a  new 
and  thorough  discussion  of  these  transits  in  1822-'24,  and  deduced  the 
value  8*58",  which  held  the  ground  for  nearly  forty  years.  About  1860 
the  researches  of  Hanscn,  Leverrier,  and  Stone  were  thought  to  have 
established  a  value  exceeding  8'90",  and  the  "  British  Nautical  Almanac  " 
used  8'95"  until  the  issue  for  1882.  In  1867  Newcomb  published  a  care- 
ful investigation,  based  upon  all  the  data  then  known,  and  deduced  the 
value  8'848ff.  Leverrier,  in  1872,  found  8-86"  from  the  planetary  per- 
turbations. The  "American  Ephemeris,"  "British  Nautical  Almanac," 
and  the  Berlin  "  Jahrbuch  "  use  Newcomb's  value,  and  the  French  "  Con- 
naissance  de  Temps"  employs  Leverrier's.  It  appears,  however,  per- 
fectly certain,  from  the  work  of  the  last  few  years,  that  the  figures  (8*80*) 
given  in  the  text  are  much  nearer  to  the  truth.  Newcomb,  in  his  "  Astro- 
nomical Constants"  (January,  1895)  gives,  as  the  final  value  based  upon 
all  available  data,  8'797"  ±  0-004.  Harkness,  in  his  "  Solar  Parallax  and 
its  related  Constants,"  deduces  as  the  result  of  a  most  exhaustive  dis- 
cussion 8-809*  ±  0'006. 


DISTANCE  AND  DIMENSIONS  OF  THE  SUN.  3f 

make  a  single  million  of  miles,  and  more  than  6,300 
years  to  traverse  the  whole. 

If  some  celestial  railway  could  be  imagined,  the 
journey  to  the  sun,  even  if  our  trains  ran  60  miles  an 
hour,  day  and  night  and  without  a  stop,  would  require 
over  175  years.  Sensation,  even,  would  not  travel  so 
far  in  a  human  lifetime.  To  borrow  the  curious  illus- 
tration of  Professor  Mendenhall,  if  we  could  imagine 
an  infant  with  an  arm  long  enough  to  enable  him  to 
touch  the  sun  and  burn  himself,  he  would  die  of  old 
age  before  the  pain  could  reach  him,  since,  according 
to  the  experiments  of  Helmholtz  and  others,  a  nervous 
shock  is  communicated  only  at  the  rate  of  about  100 
feet  per  second,  or  1,637  miles  a  day,  and  would  need 
more  than  150  years  to  make  the  journey.  Sound 
would  do  it  in  about  14-  years  if  it  could  be  transmitted 
through  celestial  space,  and  a  cannon-ball  in  about  9,  if 
it  were  to  move  uniformly  with  the  same  speed  as  when 
it  left  the  muzzle  of  the  gun.  If  the  earth  could  be 
suddenly  stopped  in  her  orbit,  and  allowed  to  fall  unob- 
structed toward  the  sun  under  the  accelerating  influence 
of  his  attraction,  she  would  reach  the  center  in  about 
two  months.  I  have  said  if  she  could  be  stopped,  but 
such  is  the  compass  of  her  orbit  that,  to  make  its  circuit 
in  a  year,  she  has  to  move  nearly  19  miles  a  second,  or 
more  than  fifty  times  faster  than  the  swiftest  rifle-ball ; 
and  in  moving  20  miles  her  path  deviates  from  perfect 
straightness  by  less  than  one  eighth  of  an  inch.  And 
yet,  over  all  the  circumference  of  this  tremendous  orbit,1 
the  sun  exercises  his  dominion,  and  every  pulsation  of 
his  surface  receives  its  response  from  the  subject  earth. 

By  observing  the  slight  changes  in  the  sun's  ap- 
parent diameter,  we  find  that  its  distance  varies  some- 
what at  different  times  of  the  year,  about  3,000,000 


38  THE  SUN. 

miles  in  all ;  and  minute  investigation  shows  that  the 
earth's  orbit  is  almost  an  exact  ellipse,  whose  nearest 
point  to  the  sun,  or  perihelion,  is  passed  by  the  earth 
about  the  1st  of  January,  at  which  time  she  is  91,385,000 
miles  distant. 

The  distance  of  the  sun  being  once  known,  its  di- 
mensions are  easily  ascertained — at  least,  within  certain 
narrow  limits  of  accuracy.  The  angular  semi-diameter 
of  the  sun  when  at  the  mean  distance  is  almost  exactly 
962",  the  uncertainty  not  exceeding  -^-^  of  the  whole. 
The  result  of  twelve  years'  observations  at  Greenwich 
(1836  to  1847)  gives  961-82*,  and  other  determinations 
oscillate  around  the  value  first  mentioned,  which  is  that 
adopted  in  the  "  American  Nautical  Almanac."  Taking 
the  distance  as  92,885,000  miles,  this  makes  the  sun's 
diameter  866,400  ;  and  the  probable  error  of  this  quan- 
tity, depending  as  it  does  both  on  the  error  of  the  meas- 
ured diameter  and  of  the  distance,  is  some  4,000  or 
5,000  miles;  in  other  words,  the  chances  are  strong  that 
the  actual  diameter  is  between  860,000  and  870,000 
miles. 

Measurements  made  by  the  same  person,  however, 
and  with  the  same  instrument,  but  at  different  times, 
sometimes  differ  enough  to  raise  a  suspicion  that  the 
diameter  is  slightly  variable,  which  would  be  nothing 
surprising  considering  the  nature  of  the  solar  sur- 
face. 

There  is  no  sensible  difference  between  the  equa- 
torial and  polar  diameters,  the  rotation  of  the  sun  on  its 
axis  not  being  sufficiently  rapid  to  make  the  polar  com- 
pression (which  must,  of  course,  necessarily  result  from 
the  rotation)  marked  enough  to  be  perceived  by  our 
present  means  of  observation. 

It  is  not  easy  to  obtain  any  real  conception  of  the 


DISTANCE  AND  DIMENSIONS  OF  THE  SUN.  39 

vastness  of  this  enormous  sphere.  Its  diameter  is  109-5 
times  that  of  the  earth,  and  its  circumference  propor- 
tional ;  so  that  the  traveler  who  could  make  the  circuit 
of  the  world  in  80  days  would  need  nearly  24  years 
for  his  journey  around  the  sun.  Since  the  surfaces  of 
spheres  vary  as  the  squares,  and  bulks  as  the  cubes,  of 
their  diameters,  it  follows  that  the  sun's  surface  is  near- 
ly 12,000  times,  and  its  volume,  or  bulk,  more  than 
1,300,000  times,  greater  than  that  of  the  earth.  If  the 
earth  be  represented  by  one  of  the  little  three-inch 
globes  common  in  school  apparatus,  the  sun  on  the  same 
scale  will  be  more  than  27  feet  in  diameter,  and  its  dis- 
tance nearly  3,000  feet.  Imagine  the  sun  to  be  hol- 
lowed out  and  the  earth  placed  in  the  center  of  the 
shell  thus  formed,  it  would  be  like  a  sky  to  us,  and  the 
moon  would  have  scope  for  all  her  motions  far  within 
the  inclosing  surface  ;  indeed,  since  she  is  only  240,000 
miles  away,  while  the  sun's  radius  is  more  than  430,000, 
there  would  be  room  for  a  second  satellite  190,000  miles 
beyond  her. 

The  mass  of  the  sun,  or  quantity  of  matter  con- 
tained in  it,  can  also  be  computed  when  we  know  its 
distance,  and  comes  out  nearly  330,000  times  as  great 
as  the  earth.  The  calculation  may  be  made  either  by 
means  of  the  proportion  given  in  the  note  to  page  33,  or 
by  comparing  the  attracting  force  of  the  sun  upon  the 
earth,  as  indicated  by  the  curvature  of  her  orbit  (about 
0-119  inch  per  second),  with  the  distance  a  body  at  the 
surface  of  the  earth  falls  in  the  same  time  under  the 
action  of  gravity,  a  quantity  which  has  been  determined 
with  great  accuracy  by  experiments  with  the  pendulum. 
Of  course,  the  fact  that  the  sun  produces  its  effect  upon 
the  earth  at  a  distance  of  93,000,000  miles,  while  a  fall- 
ing body  at  the  level  of  the  sea  is  only  about  4,000 


40  THE  SUN. 

miles  from  the  center  of  the  attraction  which  produces 
its  motion,  must  also  enter  into  the  reckoning/"* 

This  mass,  if  we  express  it  in  pounds  or  tons,  is  too 
enormous  to  be  conceived  :  it  is  2  octillions  of  tons  — 
that  is,  2  with  27  ciphers  annexed  ;  it  is  nearly  750 
times  as  great  as  the  combined  masses  of  all  the  planets 
and  satellites  of  the  solar  system  —  and  Jupiter  alone  is 
more  than  300  times  as  massive  as  the  earth.  The  sun's 
attractive  power  is  such  that  it  dominates  all  surround- 
ing space,  even  to  the  fixed  stars,  so  that  a  body  at  the 
distance  of  our  nearest  stellar  neighbor,  a  Centauri, 
which  is  more  than  200,000  times  remoter  than  the  sun, 
could  free  itself  from  the  solar  attraction  only  by  dart- 
ing away  with  a  velocity  of  more  than  300  feet  per  sec- 
ond, or  over  200  miles  an  hour  ;  unless  animated  by  a 
greater  velocity  than  this,  it  would  move  around  the 
sun  in  a  closed  orbit  —  an  ellipse  of  some  shape,  or  a 
circle  —  with  a  period  of  revolution  which,  in  the  smallest 
possible  orbit,  would  be  about  31,600,000  years,  and  if 
the  orbit  were  circular,  would  be  nearly  90,000,000. 
We  say  it  would  revolve  thus  —  that  is,  of  course,  unless 

*  The  calculation  of  the  sun's  mass,  from  the  data  given,  proceeds  aa 
follows  :  Let  M  =  the  sun's  mass,  and  m  that  of  the  earth  ;  R  =  the  dis- 
tance from  the  earth  to  the  sun,  and  r  the  mean  radius  of  the  earth  ;  T, 
the  length  of  the  sidereal  year,  reduced  to  seconds  ;  and  £  g  the  distance 
a  body  falls  in  a  second  at  the  earth's  surface.  Now,  the  distance  the 
earth  falls  toward  the  sun  in  a  second,  or  the  curvature  of  her  orbit  in  a 

n    2p 

second,  is  equal  to  —  —  (about  0'119  inch).    Hence,  by  the  law  of  gravita- 
m      M 


In  this  formula  make  tc  =  3-14169;  R,  92,900,000  miles;  T  =  31,- 
558,149-3  seconds;  r  =  3,958-2  miles;  and  £$r  =  0-0061035  mile  (16-113 
feet),  and  we  shall  get  the  result  given  in  the  text,  viz.,  M  =  330,000  m 
(nearly). 


DISTANCE  AND   DIMENSIONS  OF  THE  SUN.  41 

intercepted  or  diverted  from  its  course  by  the  influence  of 
some  other  sun,  as  it  probably  would  be.  And  we  may 
notice  here  that  in  many  cases  certainly,  and  in  most 
cases  probably,  the  stars  are  flying  through  space  at  a  far 
swifter  rate,  with  velocities  of  many  miles  per  second. 

As  for  the  attraction  between  the  sun  and  earth,  it 
amounts  to  thirty-six  hundred  quadrillions  of  tons :  in 
figures,  36  followed  by  seventeen  ciphers.  On  this 
point  we  borrow  an  impressive  illustration  from  a  care- 
ful calculation  by  Mr.  C.  B.  Warring.  We  may  imagine 
gravitation  to  cease,  and  to  be  replaced  by  a  material 
bond  of  some  sort,  holding  the  earth  to  the  sun  and 
keeping  her  in  her  orbit.  If  now  we  suppose  this  con- 
nection to  consist  of  a  web  of  steel  wires,  each  as  large 
as  the  heaviest  telegraph-wires  used  (No.  4),  then  to 
replace  the  sun's  attraction  these  wires  would  have  to 
cover  the  whole  sunward  hemisphere  of  our  globe  about 
as  thickly  as  blades  of  grass  upon  a  lawn.  It  would  re- 
quire nine  to  each  square  inch.  Putting  it  a  little  dif- 
ferently, the  attraction  between  the  sun  and  earth  is 
equal  to  the  breaking  strain  of  a  steel  rod  about  3,000 
miles  in  diameter. 

If  we  calculate  the  force  of  gravity  at  the  sun's  sur- 
face, which  is  easily  done  by  dividing  its  mass,  330,000, 
by  the  square  of  109^  (the  number  of  times  the  sun's 
diameter  exceeds  the  earth's),  we  find  it  to  be  27^  times 
as  great  as  on  the  earth  ;  a  man  who  on  the  earth  would 
weigh  150  pounds,  would  there  weigh  nearly  two  tons ; 
and,  even  if  the  footing  were  good,  would  be  unable  to 
stir.  A  body  which  at  the  earth  falls  a  little  more  than 
16  feet  in  a  second  would  there  fall  443.  A  pendulum 
which  here  swings  once  a  second  would  there  oscillate 
more  than  five  times  as  rapidly,  like  the  balance-wheel 
of  a  watch — quivering  rather  than  swinging. 


42  THE  SUN. 

Since  the  sun's  volume  is  1,300,000  times  that  of  the 
earth,  while  its  mass  is  only  330,000  times  as  great,  it 
follows  at  once  that  the  sun's  average  density  (found  by 
dividing  the  mass  by  the  volume)  is  only  about  one 
quarter  that  of  the  earth.  This  is  a  fact  of  the  utmost 
importance  in  its  bearing  upon  the  constitution  of  this 
body.  As  we  shall  see  hereafter,  we  know  that  certain 
heavy  metals,  with  which  we  are  familiar  on  the  earth, 
enter  largely  into  the  composition  of  the  sun,  so  that, 
if  the  principal  portion  of  the  solar  mass  were  either 
solid  or  liquid,  its  mean  density  ought  to  be  at  least  as 
great  as  the  earth's  ;  especially  since  the  enormous  force 
of  solar  gravity  would  tend  most  powerfully  to  compress 
the  materials.  The  low  density  can  only  be  accounted 
for  on  the  supposition,  which  seems  fairly  to  accord 
also  with  all  other  facts,  that  the  sun  is  mainly  a  ball  of 
gas,  or  vapor,  powerfully  condensed,  of  course,  in  the 
central  portion  by  the  superincumbent  weight,  but  pre- 
vented from  liquefaction  by  an  exceedingly  high  tem- 
perature. And,  on  the  other  hand,  it  could  be  safely 
predicted  on  physical  principles  that  so  huge  a  ball  of 
fiery  vapor,  exposed  to  the  cold  of  space,  would  present 
precisely  such  phenomena  as  we  find  by  observation  of 
the  solar  surface  and  surroundings. 


CHAPTFTt  II. 

METHODS  AND  APPARATUS  FOR  STUDYING    THE  SURFACE  OF 

THE  SUN. 

Projection  of  Solar  Image  upon  a  Screen. — Carrington's  Method  of  de- 
termining  the  Position  of  Objects  on  the  Sun's  Surface. — Solar  Pho- 
tography.— Photoheliographs. — Janssen's  Photographs. — Telescope 
with  Silvered  Object-Glass. — HerschePs  Solar  Eyepiece. — The  Polar- 
izing Eyepiece. 

THE  heat  and  light  of  the  sun  are  so  intense  that 
peculiar  instruments  and  methods  are  necessary  for  the 
observation  of  his  surface.  The  appliances  used  in  the 
study  of  the  moon,  planets,  and  stars  will  not  answer 
at  all  for  solar  work. 

A  very  excellent  method  of  proceeding  where  the 
object  is  to  secure  a  general  view  of  the  sun,  without 
regard  to  delicate  detail,  and  to  determine  easily  and 
rapidly  the  positions  of  spots  and  other  objects  on  the 
sun's  disk,  is  to  project  his  image  upon  a  sheet  of  card- 
board by  means  of  a  telescope. 

For  this  purpose  things  are  arranged  as  indicated  in 
the  figure.  The  sheet  of  paper  upon  which  the  image 
is  to  be  thrown  is  supported  in  front  of  the  eyepiece  by 
a  light  framework  attached  to  the  telescope.  The  dis- 
tance of  the  screen  from  the  eyepiece  depends  upon  the 
size  of  image  desired  and  the  power  of  the  eyepiece ;  a 
diameter  of  from  six  inches  to  a  foot  being  generally 
most  convenient.  Another  screen  is  usually  fitted  on 


44  THE  SUN. 

the  object-glass  end  of  the  telescope  to  balance  the  first, 
and  shade  it  from  all  light  except  tliat  which  has  passed 
through  the  instrument.  If  the  apparatus  is  to  be  used 
to  determine  the  position  of  spots  on  the  sun,  the  sur- 
face which  receives  the  image  must  be  carefully  ad- 
justed so  as  to  be  perpendicular  to  the  optical  axis  of 
the  telescope. 

1'is.  7. 


To  determine  the  position  of  objects  on  the  sun's 
disk,  Carrington  used  two  lines,  ruled  at  right  angles  to 
each  other  upon  the  screen,  and  set  at  an  angle  of  about 
45°  with  the  north  and  south  line  or  hour-circle.  The 
observations  needed  to  determine  the  place  of  a  spot 
on  the  sun's  disk  then  consist  merely  in  noting  with  a 
watch  as  accurately  as  possible  the  four  moments  at 
which  the  edge  of  the  sun's  image  crosses  the  two  lines 
(the  telescope  being,  of  course,  firmly  fixed  during  the 
whole  time),  and  the  two  moments  when  the  spot  passes 


METHODS  FOR  STUDYING  THE  SURFACE  OF  THE  SUN.     45 

them.  From  these  six  observations,  with  the  help  of 
the  data  given  in  the  almanac,  the  distance  and  direc- 
tion of  the  spot  from  the  sun's  center  may  readily  be 
calculated  by  formulae  which  would  hardly  be  suited  to 
these  pages,  but  which  may  be  found  in  the  monthly 
notices  of  the  Royal  Astronomical  Society,  vol.  xiv, 
page  153.  Fig.  8  illustrates  this  arrangement. 


FIG.  8. 


Another  method,  more  convenient  as  involving  no 
calculation,  but  less  accurate,  is  to  use  Mr.  A.  Thom- 
son's "Charts  for  Sun-spot  Observations,"  which  are 
given  in  Sir  Robert  Ball's  "  Atlas  of  Astronomy " 
(Appletons,  New  York).  Both  these  methods  require, 
however,  the  use  of  a  telescope  equatorially  mounted. 
With  an  instrument  not  so  mounted,  fairly  good  results 
may  be  obtained  by  drawing  upon  the  screen  a  circle 
with  a  diameter  about  half  that  of  the  field  of  view,  and 
noting  the  instants  when  the  edge  of  the  sun  becomes 
tangent  to  the  circle,  and  when  the  spot  crosses  it. 

With  a  small  telescope  thus  fitted  up,  one  is  in  a 
position  to  make  observations  of  real  value  as  to  the 
number,  position,  and  motions  of  the  solar  spots.  Oc- 
casionally, also,  when  the  air  happens  to  be  in  good  con- 
dition, a  considerable  amount  of  detail  can  be  made  out 
by  this  method  in  the  spots  and  upon  the  solar  surface 
generally.  The  darkening  of  £he  edge  of  the  sun, 


46  THE  SUN. 

caused  by  the  absorption  of  the  solar  atmosphere,  is 
very  noticeable,  and  the  faculse  are  conspicuous.  One 
great  advantage  of  the  method  is,  of  course,  that  several 
persons  can  thus  observe  together.  A  teacher,  for  in- 
stance, can  in  this  way  exhibit  to  a  class  of  a  dozen  all 
the  principal  features  of  the  sun's  surface,  and  be  sure 
that  they  all  see  the  things  he  desires  them  to  notice. 

Should  any  amateur  happen  to  find  upon  the  sun's 
disk  a  small,  round  spot,  which  he  has  reastfn  to  think 
is  an  intra-Mercurial  planet,  a  few  observations  of  the 
sort  indicated  above,  repeated  at  intervals  of  some  min- 
utes, would  settle  the  question  immediately,  and  give  a 
reasonably  accurate  determination  of  the  rate  and  direc- 
tion of  movement. 

If  the  instrument  has  an  equatorial  mounting  and 
clockwork,  so  that  the  image  remains  apparently  sta- 
tionary upon  the  screen,  a  very  satisfactory  tracing  can 
be  made  upon  paper  ruled  in  squares,  showing  pretty 
accurately  the  position  and  magnitude  of  all  visible 
spots,  in  a  form  suitable  to  file  away  for  reference.  The 
observations  of  Carrington's  great  work  upon  the  solar 
spots  were  for  the  most  part  made  in  this  manner. 

Of  late  years  photography  has  been  extensively  util- 
ized for  observations  of  this  sort  The  apparatus  con- 
sists of  a  telescope  fitted  with  a  camera-box  in  place  of 
an  eyepiece,  and  with  an  arrangement  for  producing  an 
instantaneous  exposure  of  the  sensitive  plate  to  the 
solar  rays. 

Since,  in  the  ordinary  achromatic  telescope,  the  rays 
which  are  most  effective  in  photographic  action  do  not 
come  to  a  focus  at  the  same  point  as  those  which  most 
strongly  affect  the  eye,  such  an  instrument,  however 
perfect  visually,  will  not  give  sharp  photographic  im- 
pressions. It  is  necessary,  for  the  best  photographic 


METHODS  FOR  STUDYING  THE  SURFACE  OF  THE  SUN.     47 

results,  to  use  object-glasses  whose  corrections  are  cal- 
culated expressly  for  the  purpose.  Mr.  Rutherfurd,  of 
New  York,  seems  to  have  been  the  first  to  appreciate 
this,  and  to  construct  an  instrument  specially  designed 
for  astronomical  photography.  To  this  end,  disdaining 
all  compromise,  he  did  not  hesitate  to  sacrifice  delib- 
erately the  visual  excellence  of  an  exquisite  object-glass 
of  thirteen  inches  diameter,  by  altering  its  curves  so  as 
to  produce  the  most  perfect  actinic  correction ;  and  he 
was  rewarded  by  a  success  until  recently  unequaled  as 
regards  the  perfection  of  the  pictures  obtained.  Some 
of  his  photographs  of  the  sun  and  moon,  obtained  about 
1866,  rival  in  sharpness  and  detail  the  drawings  of  ac- 
complished observers. 

Another  and  simpler  method  of  obtaining  the  de- 
sired corrections,  originally  tried  by  Mr.  Rutherfurd 
and  rejected  as  not  absolutely  the  best  possible,  has 
been  revived  and  used  by  Cornu,  of  Paris.  It  consists, 
not  in  regrinding  the  two  lenses  which  compose  the 
object-glass,  but  merely  in  separating  them  slightly — 
half  an  inch  or  so  for  an  instrument  of  ten-feet  focus. 
The  approximate  correction,  thus  produced,  gives  excel- 
lent results,  and  the  instrument  is  not  spoiled  for  other 
work,  since  it  requires  only  a  few  minutes  to  restore 
the  glasses  to  their  visual  adjustment. 

In  a  reflecting  telescope  there  is,  of  course,  no  diffi- 
culty of  this  sort,  since  rays  of  different  wave-length  and 
color  are  not  dispersed  by  reflection  as  by  refraction. 
Other  and  still  more  serious  difficulties,  however,  exist, 
depending  upon  the  extreme  sensitiveness  of  the  reflec- 
tor to  the  distorting  influence  of  variations  of  tempera- 
ture ;  so  that,  hitherto,  reflectors  have  not  equaled  re- 
fractors in  the  excellence  of  their  photographic  work. 
They  have  been  employed  with  very  good  success,  how- 


48  THE  SUN. 

ever,  on  several  occasions  for  the  photography  of  solai 
eclipses. 

With  telescopes  of  considerable  size  the  picture  is 
generally  formed  directly  at  the  focus  of  the  object- 
glass  without  further  enlargement.  This  is  the  case 
with  the  pictures  made  by  Mr.  Rutherfurd,  in  which 
the  diameter  of  the  sun's  image  is  about  If  inch. 


FIG.  9. 


KEW  PHOTOHELIOGRAPH. 


Copies  of  the  negatives  are  afterward  made  if  desired 
on  a  larger  scale.  In  smaller  instruments,  such  as  the 
well-known  photoheliograph  of  the  Kew  Observatory, 
an  enlarging  eyepiece  is  used,  so  constructed  as  to  dis- 
tort as  little  as  possible  the  image  formed  by  the  object- 
glass  while  magnifying  it  to  a  diameter  of  three  or  four 


METHODS  FOR  STUDYING  THE  SURFACE  OF  THE  SUN.     49 

inches.  In  this  instrument,  of  which  we  give  a  figure, 
the  diameter  of  the  object-glass  is  only  3-J-  inches,  and 
its  focal  length  50  inches;  the  tube,  instead  of  being 
conical  as  usual  and  larger  at  the  object-end,  is  made 
pyramidal  and  larger  at  the  bottom,  in  order  to  ac- 
commodate the  plate-holder  more  conveniently.  The 
whole  is  mounted  equatorially,  and  driven  by  clockwork. 
It  was  constructed  in  1857,  under  the  directions  and 
after  the  designs  of  Mr.  De  La  Rue,  and  proved  itself  a 
most  efficient  and  excellent  instrument.  A  number  of 
other  very  similar  instruments  have  since  been  made 
with  slight  improvements.  Those  employed  by  the 
English  and  Russian  parties  in  their  photographic  opera- 
tions at  the  transit  of  Venus  in  1874  were  of  this  type. 
So  also  were  those  of  the  German  parties,  except  that 
they  had  considerably  larger  telescopes,  with  apertures 
of  from  six  to  eight  inches.  The  photoheliograph  now 
used  at  Greenwich  in  maintaining  the  daily  record  of 
the  sun's  surface  is  one  of  the  Transit  of  Yenus  instru- 
ments, having  a  four-inch  object-glass,  and  giving  a  solar 
image  four  inches  in  diameter.  It  is  mounted  equato- 
rially with  clockwork.  The  instruments  at  Mauritius 
and  Dehra  Dun  are  similar.  More  recently  Greenwich 
has  come  into  possession  of  a  nine-inch  photoheliograph, 
which  is  used  in  connection  with  the  other  to  obtain 
pictures  on  a  larger  scale. 

The  sunlight  is  so  powerful  that  the  exposure  of 
the  plate  has  to  be  made  practically  instantaneous.  The 
apparatus  by  which  this  is  effected  varies  greatly  in  de- 
tail in  instruments  of  different  types,  but  in  all  cases 
consists  essentially  of  a  slide,  carrying  in  it  a  slit  of 
adjustable  width  and  capable  of  being  shot  across  in 
front  of  the  sensitive  plate  by  a  strong  spring.  At  the 
moment  of  exposure  a  trigger  or  telegraphic  key  is 
5 


50 


THE  SUN. 


touched  by  the  operator,  and  the  slide,  previously  drawn 
back  and  locked  by  suitable  mechanism,  is  released,  and 
in  its  flight  allows  the  rays  to  gleam  through  the  aper- 
ture for  a  time,  which  in  different  instruments  varies 
from  ^  to  -g-oVu-  of  a  second,  according  to  the  size  of 
the  instrument,  the  sensitiveness  of  the  collodion,  and 
the  clearness  of  the  atmosphere. 

We  give  a  figure  of  VogeFs  exposure-slide,  which  is 
perhaps  as  good  as  any.    M  is  an  electro-magnet,  which, 

FIG.  10. 


VOGEL'S  EXPOSUBE-SLIDE. 

on  the  touch  of  a  telegraph-key  in  the  observer's  hand, 
attracts  the  armature  B,  thus  releasing  the  catch  C,  and 
allowing  the  spring  S,  by  the  intervention  of  the  cord 
and  pulley,  to  draw  the  slide  containing  the  slit  A  swift- 
ly across  the  orifice  through  which  the  rays  enter  the 
camera. 

The  character  of  the  picture  produced  depends  very 
greatly  upon  the  proper  timing  of  the  exposure.  If 
the  intention  be  to  secure  an  image  of  the  sun  with 
hard,  firm  edges  from  which  measurements  can  be  made 
to  determine  the  position  of  objects  on  the  solar  disk — 


METHODS  FOR  STUDYING  THE  SURFACE  OF  THE  SUN.     51 

as  was  the  case  at  the  transit  of  Yenus — then  a  relatively 
long  exposure  is  needed ;  but  it  is  to  be  remembered 
that  the  diameter  of  the  sun's  image  increases  very  per- 
ceptibly with  lengthening  exposure,  so  that  this  diam- 
eter can  never  be  safely  used  to  furnish  the  scale  of 
measurement.  If,  on  the  other  hand,  what  is  desired 
is  a  picture  full  of  detail,  showing  the  faculae  and  the 
structure  of  the  spots,  the  exposure  must  be  greatly 
shortened  by  narrowing  the  slit  or  giving  the  slide  a 
greater  velocity ;  and  it  must  be  added,  unfortunately, 
that  the  exposure  which  brings  out  perfectly  the  cen- 
tral portions  of  the  disk  is  altogether  too  short  for  the 
portions  near  the  limb,  where  the  actinic  power  is  very 
greatly  diminished. 

This  circumstance  detracts  considerably  from  the 
.value  of  the  photographic  method.  The  skillful 
draughtsman  can  show  in  the  same  picture  details  dif- 
fering to  any  extent  in  intensity,  while  the  photograph 
is,  so  to  speak,  limited  to  the  reproduction  of  only  one 
certain  class  of  details  at  a  time.  Still  we  can  always 
be  sure  that,  whatever  a  photograph  does  show,  is  an 
autographic  representation  of  fact,  and  not  a  figment  of 
the  imagination.  This  is  not  the  case  with  drawings ;  for 
it  is  remarkable  how  widely  two  conscientious  artists 
will  differ  in  their  representations  of  the  same  object, 
seen  by  both  with  the  same  telescope,  and  under  the 
same  circumstances.  As  an  accurate  record  of  the  num- 
ber, position,  and  magnitude  of  the  solar  spots  at  any 
given  time,  the  photograph  is,  of  course,  unexception- 
able. 

Such  a  record  was  maintained  by  the  Kew  photo- 
heliograph  for  fourteen  years — from  1858  to  1872 — 
when  the  work  was  discontinued.  An  almost  equally 
important  series  of  photographs  was  kept  up  for  many 


52  TE  SUN. 

years  at  Wilna,  in  Russia,  until  the  burning  of  the  ob- 
servatory in  1877.  Since  1873  the  Kew  series  has  been 
continued  at  Greenwich,  at  least  two  pictures  being 
taken  every  day  when  the  weather  will  permit,  and 
more  than  two  if  anything  of  special  interest  demands 
it.  This  Greenwich  record  is  supplemented  by  the  neg- 
atives taken  at  Dehra  Dun,  in  India,  and  at  Mauritius. 
Taken  together  with  the  Greenwich  plates,  they  furnish 
a  practically  continuous  record  of  the  condition  of  the 
solar  surface.  At  the  same  time  there  are  occasional 
breaks,  which  might  be  remedied  if  we  had  one  or  two 
photoheliographs  on  the  American  side  of  the  Atlantic. 

Of  late,  Janssen,  at  the  new  French  physical  ob- 
servatory at  Meudon,  has  carried  solar  photography  to 
a  point  far  beyond  any  previous  attainment.  He  has 
accomplished  it  mainly  by  utilizing  the  fact  that  there 
exists  in  the  spectrum,  near  the  Fraunhofer  line  G,  a 
narrow  band  of  rays  which  possess  a  photographic  ac- 
tivity upon  the  salts  of  silver  much  more  intense  than 
that  of  any  other  portion  of  the  spectrum.  It  is  so 
intense,  indeed,  that  if  the  exposure  be  very  short  and 
properly  regulated,  the  effect  is  practically  the  same  as 
if  the  sunlight  were  monochromatic,  consisting  of  these 
rays  alone:  any  defect  in  the  color-correction  of  the 
object-glass  is  rendered  almost  harmless.  This  makes 
it  possible  to  use  an  ordinary  achromatic  object-glass, 
roughly  corrected  for  photographic  work  by  merely 
separating  the  lenses  a  trifle,  according  to  Cornu's  plan. 

With  a  five-inch  telescope  and  a  suitable  enlarging 
lens,  Janssen  produces  pictures  even  half  a  metre  in 
diameter,  and  of  extreme  perfection  in  their  delinea- 
tion of  the  details  of  the  solar  surface.  The  exposure, 
ranging  from  -g-J-g-  to  y^Vir  of  a  second,  according  to  the 
clearness  of  the  air  and  the  altitude  of  the  sun,  is  effected 


METHODS  FOR  STUDYING  THE  SURFACE  OF  THE  SUN.    53 

by  a  slide  closely  resembling  VogePs.  The  impression 
obtained  is  very  feeble,  and  requires  prolonged  and 
careful  development ;  but,  when  at  last  fairly  brought 
out,  is  every  way  admirable.  Some  very  interesting 
results,  which  we  shall  deal  with  later,  have  already 
been  deduced  from  his  plates. 

Photography,  however,  is  not  adequate  as  yet  to  the 
study  of  the  most  delicate  details  of  the  solar  surface. 
For  this  purpose  nothing  can  take  the  place  of  ocular 
observation  by  experienced  and  skillful  observers,  armed 
with  powerful  telescopes  and  suitable  appliances,  and 
on  the  watch  for  the  few  favorable  moments  when  the 
atmospheric  conditions  will  permit  successful  work. 

The  instrument  must  be  provided  with  some  form 
of  solar  eyepiece  expressly  adapted  to  the  purpose. 
The  old-fashioned  way  was  to  use  an  ordinary  eyepiece, 
fitted  with  a  dark  glass  next  the  eye.  If  the  whole 
aperture  of  a  telescope  of  any  size  is  used,  the  heat  at 
the  focus  is  so  great  as  to  endanger  the  lenses,  and  ac- 
cordingly it  was  customary  to  "  cap  down  "  the  object- 
glass — i.  e.,  to  put  on  a  cover  with  a  small  hole  in  the 
center,  so  as  to  reduce  the  aperture  to  two  or  three 
inches.  In  this  way,  of  course,  the  heat  and  light  are 
easily  diminished  to  almost  any  extent,  but  the  defini- 
tion is  greatly  injured.  According  to  well-known  opti- 
cal principles,  the  image  of  a  luminous  point  is  not  a 
point,  even  in  an  absolutely  perfect  telescope,  but,  in 
consequence  of  the  so-called  "  diffraction  "  due  to  the 
interference  of  light,  becomes  a  small  disk,  surrounded 
by  a  series  of  concentric  luminous  rings ;  the  smaller 
the  aperture  of  the  telescope,  the  larger  the  disk  with  a 
given  magnifying  power.  Similarly,  the  image  of  a 
luminous  line  is  not  a  line,  but  a  stripe  of  determinate 
width  with  fringes  on  each  side.  It  is  easy  to  see, 


54  THE   SUN. 

therefore,  that  a  telescope  of  small  aperture  can  not 
possibly  be  made  to  show  as  delicate  details  as  one  of 
larger  diameter,  and,  to  get  the  best  results  in  examin- 
ing the  surface  of  the  sun,  we  must  find  some  way  of 
diminishing  the  light  and  heat  without  cutting  down 
the  diameter  of  the  object-glass  (or  mirror,  if  we  are 
using  a  reflecting  telescope). 

A  reflecting  telescope  whose  mirror  is  of  unsfl/vered 
glass  effects  this  very  beautifully.  The  unsilvered  sur- 
face reflects  only  about  -^  of  the  incident  light  and 
heat,  and  although  the  resulting  image  is  still  too  bright 
for  the  unprotected  eye,  the  heat  is  not  troublesome, 
and  only  a  very  thin  shade-glass  is  needed.  Another 
excellent  method  is  to  silver  by  Liebig's  or  some  analo- 
gous process  the  front  surface  of  the  object-glass  of  a 
refractor.  The  silver  film  can  be  deposited  of  such  a 
thickness  as  to  allow  any  desired  percentage  of  the 
light  to  pass,  while  the  rest  is  reflected  and  not  allowed 
to  enter  the  instrument  at  all.  The  image  formed  in 
this  way  is  slightly  tinged  with  blue,  but  is  beautifully 
sharp  and  steady,  there  being  a  great  advantage  in  pre- 
venting the  heating  of  the  air  in  the  telescope-tube, 
which  occurs  with  every  other  form  of  instrument. 
The  telescopes  employed  by  the  French  parties  in  the 
observation  of  the  transit  of  Yenus  in  1874  were  pre- 
pared in  this  way.  With  its  great  advantages,  however, 
the  method  has  on  the  whole  quite  as  great  disadvan- 
tages, as  was  evident  at  Saigon,  where  clouds  were  so 
thick  that  nothing  could  be  seen  through  the  silver  film, 
and  the  observer  had  to  rub  it  off  with  a  cloth  before 
he  could  do  anything.  Then,  of  course,  a  telescope  pre- 
pared in  this  way  can  not  be  used  for  any  other  pur- 
pose. The  common  practice,  therefore,  is,  not  to  adapt 
the  instrument  for  solar  observation  by  doing  anything 


METHODS  FOR  STUDYING  THE  SURFACE  OF  THE  SUN.     55 

to  its  object-glass  or  mirror,  but  to  accomplish  the 
desired  result  by  some  modification  or  accessory  of  the 
eyepiece. 

One  of  the  best  known  and  most  generally  useful 
eyepieces  is  that  devised  by  Sir  John  Herschel,  and 
bearing  his  name.  It  is  represented  in  Fig.  11,  which 
gives  a  section  of  it.  The  light  entering  at  O  encoun- 

FlG.  11. 


SOLAR  EYEPIECE. 

ters  a  prism  of  glass,  whose  first  surface  is  placed  at  an 
angle  of  45°.  The  greater  part  of  the  light,  something 
over  ^f,  passes  through  the  prism,  emerging  perpen- 
dicular to  its  second  surface,  and  goes  out  through  the 
open  end  of  the  tube ;  the  reflected  light,  about  •£$  of 
the  whole,  is  thrown  upward  through  the  eyepiece 
proper,  A  B,  which  is  precisely  the  same  as  ordinarily 
used.  In  this  way  most  of  the  light  and  heat  are  got 
rid  of;  too  much,  however,  still  passes  the  lenses  for 
the  eye  to  bear,  and  it  is  necessary  to  use  a  shade- 
glass  ;  but  this  may  be  very  light.  The  brightness  of 
the  sun  varies  so  much  at  different  altitudes  and  under 
different  conditions  of  the  atmosphere,  that  it  is  de- 


56 


THE  SUN. 


sirable  to  have  the  thickness  of  the  shade-glass  adjust- 
able.  This  is  easily  managed  by  using  a  long,  thin 
wedge  of  dark  glass,  compensated  by  a  corresponding 
wedge  of  ordinary  glass,  and  set  in  a  proper  frame,  as 
represented  in  Fig.  12.  The  shade-glass  should  not  be 

FIG.  12. 


colored,  but  of  neutral  tint,  so  that  objects  on  the  sun's 
surface  may  be  seen  of  their  proper  hue.  The  glass 
known  as  "  London  smoke "  very  nearly  fulfills  this 
condition,  and  with  a  shade  of  this  material  the  appa- 
ratus is  exceedingly  satisfactory,  and  quite  sufficient 
for  all  ordinary  work. 

Still  finer  results,  however,  may  be  obtained  with 
more  complicated  and  expensive  "  helioscopes,"  as  they 
are  called,  which  by  means  of  polarization  reduce  the 
light  to  such  a  degree  that  no  shade  is  needed,  and9 
moreover,  enable  us  to  graduate  the  light  as  we  please  by 
merely  turning  a  milled  head.  There  are  several  forms 
of  the  apparatus  :  we  give  a  figure  of  one  constructed  by 
Merz,  slightly  modified,*  which  is  perhaps  as  convenient 
and  effective  as  any.  The  light  entering  at  A  first  en- 

*  The  modification  consists  in  substituting  the  prisms  P l  and  P 2  for 
simple  reflectors  of  black  glass,  which  are  very  apt  to  be  broken  by  the 
heat  of  the  sun's  image. 


METHODS  FOR  STUDYING  THE  SURFACE  OF  THE  SUN.     5? 

counters  the  surface  of  a  prism,  P  *,  set  at  the  polar- 
izing angle  ;  about  i|  of  the  light  passes  through  the 
prism,  emerging  perpendicular  to  its  rear  surface,  and, 
being  rejected:  about ^  is  reflected  and  polarized  by 
the  reflection.  The  reflected  ray  next  strikes  the  sur- 
face of  a  second  prism,  P a,  and  here  a  considerable  por- 
tion of  the  remaining  light  is  thrown  away.  That  which 

FIG.  18. 


Manx's  HELIOSCOPE. 

is  left  is  reflected  into  the  upper  portion  of  the  eyepiece 
parallel  to  its  original  direction,  through  an  opening  in 
the  top  of  the  circular  case  in  which  the  two  prisms 
are  mounted.  The  upper  case  is  attached  to  the  lower 
in  such  a  manner  that  it  can  be  turned  around  the  line 
C  D  as  an  axis.  It  contains  two  plane  mirrors  of  black 
glass,  placed  as  shown  in  the  figure.  With  things  in 


58  THE  SUN. 

the  position  indicated,  a  beam  of  considerable  strength 
would  reach  the  eye  at  B — so  strong,  in  fact,  as  to  be 
painful ;  and  the  same  would  be  the  case  if  the  upper 
piece  were  turned  180°,  bringing  the  mirrors  into  the 
position  shown  by  the  dotted  lines,  with  the  issuing  ray 
in  the  prolongation  of  the  incident.  But,  by  turning 
the  upper  piece  one  quarter  of  a  revolution,  the  issuing 
ray  can  be  entirely  extinguished,  and,  by  turning  it  less 
or  more  than  90°,  the  intensity  of  the  light  can  be  con- 
trolled at  pleasure.  As  no  shade-glass  is  used,  every- 
thing is  seen  of  its  proper  tint.  Another  advantage  is, 
that  there  is  no  such  disturbance  of  the  orientation  of 
the  solar  image  as  happens  with  every  form  of  diagonal 
eyepiece.  North,  south,  east,  and  west  fall  in  their 
usual  and  natural  places — a  matter  of  some  importance 
as  regards  the  convenience  of  observation. 

Still  other  forms  of  helioscopic  eyepiece  depending 
upon  polarization  have  been  devised  by  Secchi,  Lang- 
ley,  Christie,  Pickering,  and  others,  each  with  its  own 
peculiar  advantages ;  our  limits,  however,  forbid  more 
extended  treatment  of  the  subject.  We  add  merely 
that  in  some  cases,  as  in  the  study  of  the  internal  struc- 
ture of  sun-spots,  it  is  found  very  advantageous  to 
adopt  the  device  of  Dawes,  and  limit  the  field  of  view 
by  a  minute  diaphragm  made  by  piercing  a  card  or 
plate  of  ivory  with  a  hot  needle ;  thus  excluding  the 
light  from  any  portion  of  the  sun's  surface  except  that 
under  immediate  observation. 


CHAPTER  III. 

THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM. 

The  Spectrum  and  Fraunhofer's  Lines. — The  Prismatic  Spectroscope ; 
Description  of  Various  Forms  and  Explanation  of  its  Operation.-— 
The  Diffraction  Spectroscope. — The  Concave-Grating. — Analyzing 
and  Integrating  Spectroscopes. — The  Telespectroscope  and  its  Ad- 
justment.— The  Spectrograph. — Explanation  of  Lines  in  the  Spec- 
trum.— Kirchhoff's  Researches  and  Laws. — The  Sun's  Absorbing 
Atmosphere  and  Reversing  Layer. — Elements  present  in  the  Sun. — 
Lockyer's  Researches  and  Hypothesis. — Basic  Lines. — Dr.  H.  Dra- 
per's Investigations  as  to  the  Presence  of  Oxygen  in  the  Sun. — 
Schuster's  Observations. — Effect  of  Motion  upon  Wave-Length  of 
Rays  and  Spectroscopic  Determinations  of  Motion  in  Line  of  Sight. 

EVER  since  the  time  of  Newton  it  has  been  known 
that  a  beam  of  white  light  is  decomposable  into  its  con- 
stituent colors  by  passing  it  through  a  prism,  and,  under 
certain  circumstances,  the  result  is  a  rainbow-tinted  band 
or  ribbon,  which  has  been  called  the  solar  spectrum. 
In  this  spectrum  Wollaston,  in  1802,  discovered  certain 
dark  shadings,  and  in  1814  Fraunhofer  again  and  inde- 
pendently discovered  the  same  thing;  and  he  so  im- 
proved his  apparatus  and  method  of  observation  as  to 
get  not  merely  indefinite  shadings,  but  clear,  sharp 
lines,  of  which  he  made  a  map,  assigning  designations 
to  many  of  the  principal  ones.  Indeed,  these  markings 
of  the  solar  spectrum  bear  his  name  to  this  day. 

He,  however,  could  not  account  for  them,  further 
than  to  show  that  they  did  not  originate  in  his  instru- 
ment nor  in  the  earth's  atmosphere;  and  it  was  not 


(50  THE  SUN. 

until  the  publication  of  the  researches  of  Kirchhoff  and 
Bunsen,  in  1859  and  1860,  that  the  scientific  world 
came  to  appreciate  their  meaning  and  importance. 

We  speak  of  the  work  of  Kirchhoff  and  Bunsen  as 
epoch-making,  and  such  was  certainly  the  case.  At 
the  same  time  the  secret  of  the  solar  spectrum  had  been, 
in  part  at  least,  divined  before  by  Stokes,  Thomson, 
and  Angstrom;  the  latter  especially,  whose  memoir, 
published  in  1853,  would  certainly  have  obtained  a 
high  celebrity  if  it  had  appeared  in  French,  English, 
or  German,  instead  of  Swedish.  Swan  and  Zantedeschi 
had  also  given  to  the  spectroscope  nearly  its  present 
form ;  and  a  number  of  other  investigators,  among 
whom  Sir  John  Herschel,  Wheatstone,  Foucault,  and 
J.  W.  Draper  deserve  special  mention,  had  each  con- 
tributed something  important  to  the  foundations  of  the 
new  science,  for  such  it  has  proved  to  be.  The  study 
of  spectra  has  opened  a  new  world  of  research,  and 
added  some  such  reach  to  our  physics  and  chemistry  as 
the  telescope  brought  to  vision. 

Of  course,  any  extended  discussion  of  the  instru- 
ments, principles,  and  methods  of  spectroscopy  would 
be  inconsistent  with  our  limits :  we  can  only  treat  the 
subject  very  briefly. 

First,  then,  as  to  the  instrument.  It  consists  usually 
of  three  parts  :  the  collimator  so  called ;  the  light-ana- 
lyzing apparatus,  which  is  sometimes  a  prism  or  train 
of  prisms,  and  sometimes  a  diffraction  grating  ;  and  the 
view-telescope.  Figure  14  shows  the  construction  of  a 
single-prism  spectroscope,  and  the  course  of  the  rays  of 
light  through  it.  The  collimator  is  simply  a  telescope 
without  an  eyepiece,  and  having  in  the  place  of  the  eye- 
piece a  narrow  slit.  This  slit  is  placed  exactly  at  the 
focus  of  the  object-glass  of  the  collimator,  so  that  rays 


THE  SPECTROSCOPE. 


61 


from  each  point  of  the  slit  become  parallel  beams  after 
passing  the  lens,  and  a  person  looking  through  the  ob- 
ject-glass, at  the  slit,  sees  it  precisely  as  if  it  were  an 
object  in  the  sky.  Optically,  the  slit  of  the  collimator 


FIG.  14. 


ARRANGEMENT  OF  PBISMATIO  SPECTROSCOPE. 

is  thus  removed  to  an  infinite  distance  ;  while,  mechani- 
cally, it  is  still  at  the  fingers'  ends,  within  reach  of  ma- 
nipulation and  adjustment.  The  collimator,  however,  is 
not  essential.  Fraunhol'er's  work  was  all  done  with 
light  admitted  through  a  slit  in  the  window-blind  at  a 
distance  of  twenty  or  thirty  feet — a  much  less  con- 
venient arrangement,  as  is  at  once  evident. 

The  view-telescope,  which,  however,  is  no  more  essen- 
tial than  the  collimator,  is  usually  a  small  telescope  with 
an  object-glass  of  the  same  size  as  that  of  the  collimator, 
and  magnifying  from  five  to  twenty  times.  Generally, 
the  collimator  and  telescope  of  astronomical  spectro- 
scopes are  from  three  quarters  of  an  inch  to  two  inches 
and  a  half  in  diameter,  and  from  six  to  forty  inches  long. 

The  light,  after  passing  the  slit  and  object-glass  of 
the  collimator,  next  strikes  the  prism  or  grating,  and 
these  two  things — the  slit  and  the  prism  or  grating — 


62  THE   SUN. 

are  really  all  that  is  essential.  In  the  case  of  a  prism 
(which  must  be  set  with  its  refracting  edge  parallel  to 
the  slit)  the  rays  are  bent  out  of  their  course,  as  shown 
in  the  figure,  and  enter  the  view-telescope,  placed  at 
the  proper  angle  to  receive  them.  Suppose,  now,  for 
a  moment,  that  the  light  admitted  at  the  slit  is  strict- 
ly homogeneous — say  red.  The  eye  at  the  view-tele- 
scope would  then  see  a  red  image  of  the  slit,  corre- 
sponding precisely  in  form  and  proportions  to  the  slit 
itself,  widening  or  narrowing  as  the  slit  is  acted  on 
by  its  adjusting  screw.  If  instead  of  a  slit  the  open- 
ing had  some  other  form,  as  an  arc  of  a  circle,  a  tri- 
angle, or  a  square,  the  image  seen  would  imitate  it, 
always  having  the  same  color  as  the  light  admitted. 
Suppose,  again,  that  the  light  is  not  homogeneous,  but 
consists  of  two  kinds  mixed  together — say  red  and  yel- 
low. Viewing  the  slit  directly,  without  the  spectro- 
scope, one  would  only  see  a  single  orange-colored  im- 
age ;  but  with  the  spectroscope  one  would  see  two 
widely  separated  images,  one  of  them  red,  the  other 
yellow.  This  is  because  the  prism  refracts  the  two 
kinds  of  light  differently,  so  that  after  the  rays  have 
passed  the  prism  they  strike  the  object-glass  of  the  view- 
telescope  in  different  directions,  and  then  make  images 
in  different  places.  If  the  light  is  composed  not  of  two 
kinds  only,  but  many,  the  images  will  be  numerous, 
ranged  side  by  side  like  the  pickets  of  a  fence ;  and  if, 
as  in  the  case  of  a  candle-flame,  the  light  emitted  con- 
tains an  indefinite  number  of  tints,  then  the  slit-images, 
placed  side  by  side,  will  coalesce  into  a  contmuous 
band  of  color.  If,  in  the  candle-light,  certain  kinds  of 
light  are  specially  abundant,  then  the  corresponding 
slit-images  will  be  more  brilliant  than  their  neighbors ; 
and  if,  as  is  usually  the  case,  the  slit  be  narrowed  to  a 


THE   SPECTROSCOPE. 


63 


line,  these  slit-images  will  become  bright  lines  in  the 
spectrum — lines  only  because  the  slit  is  itself  a  line, 
which,  of  course,  is  the  best  form  to  give  the  light-ad- 
mitting aperture,  in  order  that  the  different  images  may 
overlap  and  interfere  as  little  as  possible. 

If  any  kinds  of  light  be  wanting,  then  the  corre- 
sponding images  of  the  slit  will  be  missing,  and  the 
spectrum  will  be  marked  by  dark  bands  or  lines. 

FIG.  15. 


BUNSEN'B  SPECTROSCOPE. 

The  cut  (Fig.  15)  shows  the  actual  appearance  of 
what  is  known  as  the  chemical  spectroscope,  ordinarily 
used  in  laboratories.  Besides  the  collimator  A,  and  the 
telescope  B,  it  has  a  third  tube  C,  which  carries  a  fine 
scale  photographed  on  glass  at  the  end  farthest  from 
the  prism.  There  is  a  lens  in  the  tube  at  the  end  next 
the  prism,  so  that  the  observer  at  the  telescope  sees  this 
scale  running  across  the  field  of  view  at  the  edge  of  the 
spectrum,  and  thus  has  the  means  of  noting  accurately 


64  THE   SUN. 

the  position  of  any  lines  he  may  find.     This  arrange- 
ment is  due  to  Bunsen. 

It  is  often  desirable  to  obtain  a  greater  separation  of 
the  different  colors — "  dispersion,"  to  use  the  technical 
term — than  a  single  prism  would  produce.  In  this  case, 
the  rays  after  passing  through  the  first  prism  may  be 
transmitted  through  a  second  and  a  third,  and  so  on, 
until  they  reach  the  view-telescope.  With  prisms  as 
commonly  made,  it  is  difficult  to  use  more  than  six  in 
this  manner,  but  it  is  possible  by  reflection  properly 
managed  to  return  the  rays  through  a  second  prism- 
train  connected  with  the  first,  so  as  to  get  the  virtual 
effect  of  from  ten  to  twelve  prisms.  The  instrument 
figured  on  pages  74  and  202,  and  used  for  observatiao 
of  the  solar  prominences,  is  of  this  kind. 


COMPOUND    PRISM. 


DIRECT-VISION    PRISM 

Another  way  is  to  use  a  compound  prism,  so  called, 
composed  of  a  very  obtuse-angled  prism,  A  B  E,  of 
Borne  highly  dispersive  material,  usually  heavy  flint 
glass,  flanked  by  two  prisms  of  lighter  glass  with 
their  refracting  angles  reversed.  Prisms  of  this  kind 


THE   SPECTROSCOPE.  65 

can  be  made  of  much  higher  dispersive  power  than 
simple  prisms,  and  of  course  a  smaller  number  will 
answer  the  same  purpose.  By  properly  proportion- 
ing the  angles  C  A  E  and  E  B  D,  it  is  possible  to  make 
the  yellow  rays  of  the  spectrum  pass  through  without 
change  of  direction,  while  still  retaining  a  considerable 
dispersion.  An  instrument  with  prisms  of  this  kind  is 
called  a  "  direct-vision  "  spectroscope,  and  in  some  cases 
is  much  more  convenient  than  the  other  forms. 

Thollon  has  recently  constructed  compound  prisms 
having  the  dense  glass  prism  replaced  by  a  chamber 
filled  with  carbon  disulphide,  which  possesses  an  enor- 
mous dispersive  power ;  with  a  train  of  these  prisms  he 
has  obtained  views  of  the  spectrum  only  equaled  by  the 
performance  of  the  best  diffraction  gratings.  A  dis- 
persion equal  to  that  of  thirty  or  forty  prisms  of  an 
ordinary  spectroscope  is  easily  reached.  The  behavior 
of  these  disulphide  prisms  is,  however,  far  from  satis- 
factory for  general  work,  since  they  are  extremely  sen- 
sitive to  small  changes  of  temperature,  which  cause 
irregular  refractions  in  the  liquid,  and  destroy  the  defi- 
nition. 

We  have  used  the  expression,  the  dispersive  power 
of  thirty  or  forty  prisms ;  but  that  is  very  indefinite, 
because  the  dispersive  power  of  a  spectroscope  depends 
upon  its  linear  dimensions  as  well  as  the  kind  and  num- 
ber of  prisms  and  is  proportional  to  the  dimensions. 
That  is  to  say,  if  a  given  spectroscope  has  the  size  of  its 
.prisms,  and  the  diameter  and  focal  lengths  of  its  col- 
limator  and  telescope  doubled,  retaining,  however,  the 
former  slit  and  eyepiece,  its  dispersive  power  will  be 
doubled  by  the  change.  Thus  a  large  single-prism  in- 
strument may  equal  in  working  power  a  small  one  of 
many  prisms. 


66  THE  SUN. 

Lord  Rayleigh  has  shown  that  the  resolving  power 
of  a  spectroscope,  constructed  with  prisms  of  a  given 
substance,  depends  upon  the  length  of  the  route  pur- 
sued bj  the  rays  of  light  in  traversing  them. 

As  has  been  said,  a  diffraction  grating  may  replace 
the  prism  in  a  spectroscope.  This  diffraction  grating 
is  merely  a  system  of  close,  equidistant,  parallel  lines 
ruled  upon  a  plate  of  glass  or  polished  speculum-metal. 
The  closer  the  lines,  the  greater  the  dispersion  pro- 
duced ;  the  larger  the  ruled  surface,  the  more  light  is  at 
the  observer's  disposal,  provided  the  collimator  and 
view-telescope  are  large  enough  to  utilize  the  whole 
ruling.  The  greater  the  total  number  of  lines,  the 
higher  the  resolving  power  of  the  grating,  or  power  of 
separating  close  lines  in  the  spectrum. 

It  hardly  needs  to  be  said  that  the  making  of  a  sat- 
isfactory grating  is  by  no  means  an  easy  matter.  To 
work  a  surface  optically  accurate,  and  to  rule  it  with 
perfectly  straight  equidistant  paralled  lines,  20,000  to  the 
inch  or  so,  and  all  of  the  same  width  and  depth,  is  one 
of  the  most  delicate  and  difficult  of  mechanical  opera- 
tions. The  first  that  were  fit  for  spectroscopic  use  were 
made  in  this  country,  about  1S71,  by  Mr.  Rutherfurd, 
of  New  York,  upon  a  ruling  machine  devised  and  con- 
structed by  him  for  the  purpose :  they  were  first  act- 
ually applied  in  solar  spectroscopy  by  the  writer,  in 
1873.  In  1881,  when  the  first  edition  of  this  book  ap- 
peared, some  very  fine  ones  had  already  been  produced, 
with  ruled  surfaces  nearly  two  inches  square,  carrying 
17,280  lines  to  the  inch.  One  of  these,  still  in  constant 
use  in  the  Princeton  Observatory,  is  especially  excellent, 
and  except  in  size  is  hardly  inferior  to  the  magnificent 
specimens  now  produced  at  Baltimore  by  Professor 
Rowland's  wonderful  machine,  which  at  present,  with 


THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.      (57 

its  newest  improvements  and  refinements,  is  quite  with- 
out a  rival,  and  almost  ideally  perfect.  Ever  since  1882 
it  has  been  turning  off  gratings  of  admirable  excel- 
lence :  the  largest  have  a  ruled  surface  of  about  5 £  by  4 
inches,  with  more  than  100,000  lines  (20,000  to  the 
inch).  These  have  been  widely  distributed  among  sci- 
entific workers,  and  it  is  not  too  much  to  say  that  all 
the  recent  important  researches  upon  the  solar  spec- 
trum (Thollon's  alone  excepted)  owe  their  success  to 
Rowland's  gratings. 

An  explanation  of  the  way  in  which  diffraction 
spectra  are  produced  by  a  grating  lies  beyond  our 
scope ;  for  this  the  reader  is  referred  to  any  good  trea- 
tise on  optics.  We  merely  state,  in  passing,  that  diffrac- 
tion has  nothing  to  do  with  refraction,  but  depends 
upon  the  fact  that  the  ether  waves,  of  which  light  con- 
sists, under  certain  circumstances  "  interfere  "  with  each 
other  and  produce  brilliant  effects  of  color.  We  say 
spectra,  because,  while  a  prism  gives  but  one  spectrum, 
a  grating  gives  many,  and  of  different  degrees  of  dis- 
persion, which  is  often  a  matter  of  much  convenience. 
Of  course,  no  one  of  the  spectra  is  as  brilliant  as  if  it 
were  the  only  one,  but  with  sunlight  this  is  a  matter  of 
small  consequence.  Besides,  it  is  possible,  by  giving  the 
proper  shape  to  the  diamond  point  which  rules  the  lines 
and  properly  regulating  the  depth  of  cut,  to  produce 
gratings  in  which  most  of  the  light  shall  be  concen- 
trated in  a  single  one  of  the  spectra  at  the  expense  of 
the  rest. 

A  good  grating  combined  with  a  suitable  collimator 
and  telescope  constitutes  a  spectroscope  which  for  most 
solar  work  is  incomparably  superior  in  power  and  con- 
venience to  any  prismatic  instrument  of  similar  dimen- 
sions, so  that  as  a  matter  of  fact  the  grating-spectro- 


68  THE  SUN. 

scope  has  almost  superseded  the  other  in  this  sort  of 
work. 

Fig.  17  shows  the  arrangement  of  the  different  parts 
of  such  an  instrument.     The  collimator  and  view-tele- 

FIG.  17. 


COLLIMATOR  P  SLIT 


DIFPKAOTION  SPECTBOSCOPE. 

scope  are  placed  with  their  object-glasses  close  together, 
and  their  tubes  making  as  small  an  angle  as  possible, 
consistently  with  keeping  the  grating  at  a  manageable 
distance,  since  both  collimator  and  telescope  must  be 
pointed  at  the  center  of  the  grating.  The  grating  is 
mounted  on  a  frame  with  an  axis  at  A,  so  that  it  can 
rotate  in  the  plane  of  the  dispersion,  the  ruled  lines 
being  parallel  to  this  axis.  The  frame  which  carries 
the  grating  must  be  so  constructed  as  to  support  it 
steadily  and  firmly,  without  the  slightest  strain,  for  it 
is  essential  to  its  good  performance  that  the  surface  be 
strictly  plane :  an  abnormal  pressure  of  a  single  ounce 
at  one  of  the  corners  will  sensibly  affect  its  perform- 
ance, and  four  ounces  would  bend  the  plate  sufficiently 
to  ruin  the  definition. 

As  the  different  orders  of  spectra  overlap  each 
other  (the  red  end  of  the  second  order  spectrum  over- 
lapping the  blue  of  the  third,  etc.),  it  is  sometimes  neces- 
sary to  separate  them,  and  this  can  be  done  in  a  man- 
ner first  suggested  by  Fraunhof  er,  by  interposing  between 


THE  SPECTROSCOPE  AND   THE   SOLAR  SPECTRUM.     69 

the  grating  and  view-telescope  a  single  prism  with  its 
plane  of  dispersion  perpendicular  to  that  of  the  grating, 
the  telescope  being  then  inclined  at  the  proper  angle  to 
receive  the  rays.  A  direct-vision  prism  in  the  eyepiece 
answers  the  same  purpose,  though  less  satisfactorily.  In 
many  cases  a  suitably  colored  shade-glass  is  sufficient. 

Fig.  18  is  from  a  photograph  of  an  instrument  act- 
ually in  use  at  Princeton  for  observations  upon  solar 
prominences.  It  is  designed  to  be  attached  to  a  nine- 
inch  equatorial,  its  collimator  and  view-telescope  being 
each  only  about  thirteen  inches  long,  with  a  diameter 
of  an  inch  and  a  quarter.  The  prism,  P,  is  used  only 

FIG.  18. 


PEINCETON  SPECTROSCOPE. 


occasionally  and  can  easily  be  removed.  The  telescope, 
T,  is  then  lowered  to  the  same  level  as  the  collimator,  so 
as  to  be  perpendicular  to  the  lines  of  the  grating. 

There  is,  however,  a  form  of  grating-spectroscope 


70  THE   SUN. 

invented  by  Professor  Rowland  which  uses  not  a  flat 
but  a  concave  grating,  and  dispenses  with  both  the  col- 
limator  and  the  telescope.  For  certain  researches,  such 
as  the  mapping  of  the  spectrum  of  the  sun  or  of  metal- 
lic vapors,  or  for  comparing  together  different  spectra, 


Fio.  19. 


CONCAVE  GRATING-SPECTBOSCOPB. 


it  is  the  most  powerful  and  effective  of  all  spectroscopic 
apparatus.  The  arrangement  is  that  indicated  in  Fig. 
19.  The  grating,  G,  is  mounted  at  one  end  of  a  stiff 
bar,  C,  at  the  other  end  of  which  is  placed  an  eyepiece. 


THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.     71 

Two  pivots,  one  with  its  center  directly  under  the  cen- 
ter of  the  ruled  surface  of  the  grating,  and  the  other 
at  I,  distant  from  the  first  by  the  radius  of  the 
spherical  surface  of  the  grating,  connect  the  rod  with 
two  carriages  which  ride  upon  the  rails  R  and  R'. 
These  are  firmly  secured  exactly  at  right  angles  to  each 
other  upon  the  two  strong  beams  A  and  B,  and  at  the 
point  where  the  rails  would  meet  is  placed  the  slit  S. 
With  this  arrangement,  if  I  is  made  to  slide  along  B 
either  toward  the  right  or  left,  G  will  slide  along  A  ;  and 
since  the  angle  at  S  is  a  right  angle,  the  three  points,  G, 
S,  and  I,  will  always  be  on  the  circumference  of  a  circle 
whose  diameter  is  G  I.  Under  these  conditions  light 
passing  through  the  slit  and  striking  the  grating  will 
form  a  perfectly  focused  spectrum  at  I,  which  can  be 
viewed  with  the  eyepiece. 

If  desired,  a  photographic  plate  can  be  placed  at  I 
and  the  spectrum  photographed.  If  I  is  moved  toward 
the  right  it  will  run  toward  the  red  end  of  the  spec- 
trum, and  toward  the  violet  if  moved  toward  the  left. 
With  a  six-inch  grating  the  rod  G  I  is  usually  from 
fifteen  to  twenty-five  feet  long,  and  the  dispersion  is 
tremendous.  The  apparatus  is  mounted  in  a  large  room 
perfectly  darkened,  and  the  sunlight  is  brought  in  by  a 
heliostat  mirror,  through  a  suitably  protected  orifice. 
It  was  with  an  apparatus  of  this  kind  that  Professor 
Rowland  made  his  great  photographic  map  of  the  solar 
spectrum  (page  78),  and  has  studied  the  spectra  of 
nearly  all  the  chemical  elements.  The  theory  of  the 
instrument  is  quite  beyond  our  range :  those  who  are 
sufficiently  versed  in  mathematics  will  find  it  given  in 
the  "  Encyclopaedia  Britannica,"  article  "  Wave-theory 
of  Light," '§  14. 

The  prismatic  and  diffraction  (or  interference)  spec- 


72  THE  SUN. 

tra  differ  from  each  other  to  a  certain  extent,  not,  of 
course,  in  the  order  of  colors  or  of  lines,  but  in  their 
relative  distances.  In  the  prismatic  spectrum  the  red 
and  yellow  portion  is  much  compressed,  while  the  violet 
is  greatly  extended ;  with  the  diffraction  spectrum  the 
reverse  is  the  case ;  the  lines  in  the  violet  are  crowded 
together,  and  those  in  the  red  are  widely  separated. 

In  the  diffraction  spectrum  the  lines  are  almost  per- 
fectly straight ;  in  the  prismatic,  generally  more  or  less 
curved;  we  say  generally,  because  there  are  forms  of 
high-dispersion  spectroscope  in  which  this  curvature  is 
corrected.  This  curvature  is  caused  by  the  fact  that 
the  rays  from  the  top  and  bottom  of  the  slit  do  not 
meet  the  refracting  surface  at  the  same  angle  as  those 
from  the  middle  of  the  slit ;  they  are,  therefore,  differ- 
ently refracted  ;  in  consequence,  the  slit-images  of  which 
the  spectrum  is  built  up  are  not  straight,  but  distorted. 

We  may  add  that  the  dark  lines  which  often  run 
lengthwise  through  the  spectrum  are  merely  due  to 
roughnesses  or  particles  of  dust  on  the  jaws  of  the  slit. 
It  is  almost  impossible  to  make  and  keep  the  edges  of 
the  slit  so  clean  and  smooth  that  lines  of  this  sort  will 
not  appear  when  the  opening  is  very  narrow. 

The  spectroscope  may  be  used  in  two  entirely  dif- 
ferent ways :  it  may  simply  have  its  collimator  pointed 
toward  the  source  of  light ;  or  a  lens  may  be  interposed 
between  the  slit  and  the  luminous  object,  so  as  to  form 
an  image  of  the  latter  on  the  slit. 

In  the  first  case,  the  instrument  is  said  to  be  an  in- 
tegrating spectroscope,  because  each  point  in  the  slit 
receives  light  from  the  whole  of  the  luminous  object,  so 
that  the  spectrum  is  alike  through  its  whole  width,  and 
represents  the  average  light  of  the  object — it  lumps  the 


THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.     73 

whole,  so  to  speak.  In  the  second  case,  different  parts 
of  the  slit  are  illuminated  by  light  from  different  parts 
of  the  object ;  the  top  of  the  slit  gets  the  light  from 
one  point,  the  middle  of  the  slit  from  another,  and  the 
bottom  from  a  third.  If,  then,  the  lights  emitted  by 
the  three  points  differ,  their  spectra  will  differ  also,  and 
the  observer  will  find  that  different  portions  of  the 
width  of  his  spectrum  will  differ  correspondingly — the 
upper  portion  will  be  unlike  the  middle,  and  the  mid- 
dle will  differ  from  the  bottom.  An  instrument  ar- 
ranged thus  is  called  an  analyzing  spectroscope,  because 
it  enables  us  to  determine  separately  the  spectra  of  vari- 
ous portions  of  an  object,  and  thus  to  analyze  its  consti- 
tution ;  as,  for  instance,  a  sun-spot  and  its  surroundings. 
For  most  purposes,  especially  astronomical,  it  is  much 
the  most  satisfactory.  Approximately  the  same  end 
may  be  reached,  in  some  cases,  by  placing  the  slit  very 
near  the  luminous  object,  as  in  flame  analysis,  but  it  is 
usually  much  more  convenient  and  better  to  use  the 
lens.  In  astronomical  work  the  object-glass  of  a  large 
equatorial  telescope  is  generally  employed  to  form  the 
image  of  the  celestial  object,  and  the  spectroscope  is 
attached  at  the  eye-end  of  the  telescope,  the  eyepiece 
being  removed.  The  combined  instrument  is  then  often 
called  a  tele-spectroscope.  Fig.  20,  on  the  next  page, 
represents  the  apparatus  long  used  at  the  Dartmouth 
College  Observatory. 

It  is  usually  very  important  that  the  slit  of  the  in- 
strument be  precisely  in  the  focal  plane  of  the  object- 
glass  of  the  telescope  for  the  rays  especially  under  ex- 
amination. On  account  of  the  so-called  "secondary 
spectrum "  of  the  achromatic  lens,  this  focal  plane  is 
quite  different  for  the  different  colors,  and  the  spectro- 
scope requires  to  be  slid  in  or  out,  so  as  to  vary  the 


THE  SUN. 


distance  of  its  slit  from  the  great  object-glass  of  the 
telescope  according  to  circumstances.  The  same  end 
may  be  obtained  (less  satisfactorily,  however)  by  a  sec- 
ond lens  between  the  object-glass  and  the  slit,  and 
pretty  near  the  latter.  By  moving  this  lens,  the  focus 


FIG.  2a 


TBLE-SPECTROSOOPB. 

can  be  made  to  fall  exactly  on  the  slit.  Neglect  of  this 
adjustment*  will  make  many  of  the  most  interesting  and 
important  spectroscopic  observations  quite  impossible. 

As  has  been  already  mentioned,  photography  is  often 
used  in  connection  with  the  spectroscope,  and  the  name 

*  See  Note  A,  page  351-355. 


THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.   f5 

spectrograpli  has  been  given  to  the  combination  instru- 
ment. Fig.  21  represents  the  spectrograph  which  is 
attached  to  the  23-inch  equatorial  of  the  Halsted  Ob- 
servatory at  Princeton.  As  shown  in  the  figure,  it  uses 
a  grating,  but  this  can,  at  pleasure,  be  exchanged  for  a 
train  of  prisms. 

Some  of  the  advantages  of  photography  are  self- 
evident — as,  for  instance,  the  quickness  and  accuracy 
with  which  a  map  of  any  portion  of  the  spectrum  can 
be  produced,  as  contrasted  with  the  tediousness  of  any 
drafting  process.  Then,  too,  since  our  modern  plates 
admit  of  any  desired  length  of  exposure  (instead  of  dry- 
ing up  in  a  few  minutes,  as  our  old  "  wet  plates  "  used 
to),  they  enable  us  to  get  satisfactory  negatives  of  spec- 
tra far  too  faint  to  be  visually  observed.  Finally,  there 
is  a  long  range  of  ultra-violet  spectra  composed  of  rays 
of  too  short  wave-length  and  too  high  a  "  pitch  "to  be 
seen  by  the  human  eye,  while  they  are  easily  photo- 
graphed. 

Per  contra,  the  ordinary  photographic  plate  is  by 
no  means  impartial — it  is  very  sensitive  to  blue  and 
purple,  and  very  obtuse  to  green,  yellow,  and  red.  The 
new  isochromatic  and  orthochromatic  plates  are,  how- 
ever, better  in  this  respect,  and  by  using  them  (com- 
bined with  a  reasonable  amount  of  patience)  it  is  now 
possible  to  work  down  even  to  the  red  extremity  of  the 
visible  spectrum. 

If  the  collimator  of  a  spectroscope  of  any  form  be 
directed  toward  an  ordinary  lamp,  or  upon  the  incan- 
descent lime  of  a  calcium-light,  the  observer  will  get 
simply  a  continuous  spectrum ;  a  band  of  color  shading 
gradually  from  the  red  to  the  violet,  without  markings 
or  lines  of  any  kind.  If  the  instrument  be  turned  to- 
ward the  sun  he  will  obtain  something  much  more  in- 


LARGE  PRINCETON  SPECTROSCOPE  (FITTED  FOK  PHOTOGRAPHY). 


THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.      77 


teresting — a  band  of  color,  as  before,  but  marked  by 
hundreds  and  thousands  of  dark  lines,  some  fine  and 
black,  like  hairs  drawn  across  the  spectrum,  while  others 
are  hazy  and  indistinct. 

Most  of  them  retain  their  appearance  and  position 
perfectly  from  day  to  day  ;  some  of  them,  however,  are 
more  intense  at  one  time  than  another,  and  when  the  sun 
is  near  the  horizon  certain  lines  in  the  red  and  yellow 
become  extremely  conspicuous,  in  such  a  way  as  to  make 
it  clear  that  they,  at  least,  have  something  to  do  with  our 

FIG.  22. 


A     a      B 


E     b 


terrestrial  atmosphere.  Fig.  22  is  a  reproduction  of  a 
portion  of  Fraunhofer's  map  of  the  solar  spectrum, 
showing  what  one  might  fairly  expect  to  see  (except 
as  to  color)  with  an  excellent  single-prism  spectroscope. 
Fig.  23  is  a  drawing  of  a  very  small  portion  of  the 
spectrum  in  the  green,  as  shown  by  a  very  powerful 
spectroscope.  The  scale  is  that  of  Angstrom's  map. 
The  large,  heavy  lines  are  known  as  the  little  I  group, 
and  are  due,  as  we  shall  soon  see,  part  of  them  to  the 
presence  of  iron  and  nickel,  and  part  to  magnesium,  as 
gases  in  the  solar  atmosphere. 

SPECTRUM   MAPS. 

There  are  numerous  maps  of  the  solar  spectrum: 
the  earliest  of  any  scientific  value  was  KirchhofFs,  which 


Y8  THE   SUN. 

appeared  in  1861-'62.  Its  scale  was  purely  arbitrary, 
and  not  even  self-consistent  throughout,  so  that  when 
Angstrom's  map  of  the  "  normal  spectrum  "  was  pub- 


Fio.  28. 


GKOTJP  IN  SOLAR  SPECTRUM. 


lished  in  1868,  made  with  a  diffraction  grating,  and 
platted  on  a  scale  of  wave-lengths  (one  unit  of  the  scale 
corresponding  to  one  ten-millionth  of  a  millimetre  of 
wave-length),  it  soon  superseded  Kirchhoff's,  and  has 
not  yet  ceased  to  be  used  for  reference.  Angstrom's 
grating  was  imperfect,  however,  and  at  present  Row- 
land's photographic  map,  dating  from  about  1890,  is 
accepted  as  "  the  standard."  It  covers  the  ultra-violet 
from  about  X*  3000,  and  extends  down  through  the 
visible  spectrum  into  the  red  to  X  6900,  just  below  the 
B  line.  It  is  unfortunate  that  it  does  not  go  lower,  but 
Mr.  Higgs,  of  Liverpool,  has  published  a  set  of  photo- 
graphs of  different  parts  of  the  spectrum,  and  two  of 
these  complete  Rowland's  map  to  the  lower  limit  of 

*  A,  is  the  symbol  now  universally  used  for  the  "  wave-length  "  of  a 
ray  of  light.  "  A  3000  "  means  that  part  of  the  spectrum  (invisible  in 
this  case)  where  the  wave-length  is  3000  ten-millionths  of  a  millimetre. 


THE   SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.      79 

photographic  capability.  In  clearness  and  beauty  of 
execution  Mr.  Higgs's  maps  surpass  everything  that 
has  been  done  in  that  line.  Thollon's  great  map  cov- 
ers only  the  lower  half  of  the  spectrum,  and  is  subject 
to  the  same  objection  as  that  of  Kirchhoff — an  arbitrary 
scale.  Its  peculiarity  is  in  presenting  the  appearance 
of  the  spectrum  corresponding  to  different  altitudes  of 
the  sun. 

As  there  are  invisible  rays  beyond  the  upper  or 
violet  end  of  the  visible  spectrum,  so  also  below  the 
red  there  is  a  still  longer  range  of  rays  of  wave-length 
too  great  for  the  human  eye  to  perceive.  Photography 
carries  us  a  little  way  into  this  infra-red  region,  but  not 
far.  For  mo*st  of  our  knowledge  of  it  we  are  dependent 
upon  the  "  bolometric  "  work  of  Professor  Langley,  to 
which  we  shall  have  to  refer  more  specially  in  treating 
of  the  heat  of  the  sun.  He  has  already  succeeded  in 
mapping  this  "  heat-spectrum,"  as  it  is  sometimes  called, 
sufficiently  to  show  that  it  is  filled  with  dark  bands  and 
lines,  and  has  fixed  the  position  of  many  of  them. 

If,  instead  of  using  the  sun  or  an  ordinary  flame  for 
the  source  of  light,  we  examine  with  the  spectroscope 
an  electric  spark,  or  the  arc  between  carbon  points,  or 
the  light  produced  by  passing  the  discharge  of  an  induc- 
tion coil  through  a  rarefied  gas,  we  shall  get  a  spectrum 
of  quite  a  different  sort — a  spectrum  consisting  of  bright 
lines  upon  a  dark  or  faintly  luminous  background  ;  and 
it  will  be  found  that  the  spectrum  developed  will  al- 
ways be  the  same  under  similar  circumstances,  depend- 
ing mainly  upon  the  material  of  the  electrodes  (the 
points  between  which  the  discharge  passes),  and  the 
nature  of  the  intervening  gas,  but  also,  to  a  certain 
extent,  upon  its  density  and  the  intensity  of  the  elec- 
tric discharge.  So,  also,  if  certain  easily  vaporized  salts 


80  THE   SUN. 

are  introduced  into  the  blue  flame  of  a  Bunsen  gas-burn- 
er,  or  of  a  spirit-lamp  even,  the  flame  becomes  colored., 
and  its  spectrum  is  a  spectrum  of  bright  lines,  which 
are  perfectly  characteristic  of  the  metal  whose  salt  is 
used.  An  ordinary  candle-flame,  indeed,  almost  always 
shows  one  such  bright  line  in  the  yellow,  as  had  been 
noticed  many  years  before  Swan,  in  185T,  showed  it  to 
be  due  to  the  presence  of  sodium,  which  in  the  form 
of  common  salt  is  universally  distributed. 

Fraunhofer,  as  early  as  1814,  had  discovered  that 
this  line  (or  lines  rather,  for  it  is  really  composed  of 
two,  easily  separated  by  a  spectroscope  of  no  great 
power)  exactly  coincides  with  the  double  line  which  he 
named  D,  in  the  solar  spectrum  ;  and  he  had  found  the 
same  line  in  the  spectra  of  certain  stars  also ;  but  he  did 
not  know  that  the  line  was  due  to  sodium,  or  in  all 
probability  he  would  have  anticipated  by  nearly  half  a 
century  the  discovery  which  lies  at  the  foundation  of 
modern  spectrum  analysis.  As  has  been  said  before, 
the  principles  involved  seem  to  have  been  more  or 
less  distinctly  apprehended  by  several  persons — Stokes 
and  Angstrom  especially — years  before  the  publication 
of  Kirchhoif  in  1859 ;  but  it  was  Kirchhoffs  work 
which  first  bore  fruit. 

It  is  not  necessary  to  repeat  here  again  the  oft-told 
story  how  he  found  that,  when  sunlight  is  made  to 
pass  through  a  flame  containing  sodium-vapor,  the  D- 
lines  in  the  spectrum  of  this  sunlight  come  out  with 
increased  intensity ;  though,  when  a  screen  is  interposed 
between  the  sun  and  the  flame,  the  lines  are  bright,  as 
usual  in  such  a  flame.  He  found,  too,  that  when  the 
incandescent  lime-cylinder  of  the  calcium-light  is  placed 
behind  the  sodium-flame,  a  precisely  similar  phenomenon 
occurs,  and  the  bright  lines  of  the  flame-spectrum  are 


THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.      81 

reversed  to  dark  ones.*     He  found  the  same  thing  to 
hold  good  also  for  a  flame  colored  by  lithium. 

The  sum  of  his  results  may  be  stated  as  follows : 

1.  Solids  and  liquids,  when  incandescent,  give  con- 
tinuous spectra ;  and,  as  we  now  know,  the  same  thing 
is  true  of  gases  also  at  great  pressures. 

2.  Bodies  in  the  gaseous  state  (and  not  compressed) 
give  discontinuous  spectra  consisting  of  bright  lines  and 
bands;  and  these  bright-line  spectra  are  different  for 
different  substances,  and  characteristic,  so  that  a  given 
substance  is  identifiable  by  its  spectrum.f 

3.  When  light  from  a  solid  or  liquid  incandescent 
body  passes  through  a  gas,  the  gas  absorbs  precisely 
those  rays  of  which  its  own  spectrum  consists ;  so  that 
the  result  is  a  spectrum  marked  by  black  lines  occupy- 
ing exactly  the  same  positions  which  would  be  held  by 
the  bright  lines  in  the  spectrum  of  the  gas  alone. 

His  conclusion,  therefore,  was  that  the  luminous  sur- 
face of  the  sun  (the  "  photosphere ")  is  composed  of 
solid  or  liquid  matter  giving  by  itself  a  purely  contin- 
uous spectrum,  and  that  the  dark  lines  which  mark  the 
spectrum  are  produced  by  the  transmission  of  the  light 
through  an  overlying  atmosphere.  He  believed  the 
photosphere  to  be  a  continuous  sheet  of  liquid — a  mol- 
ten ocean — but  numerous  facts  since  learned  make  it 
almost  certain  that  it  is  rather  a  sheet  of  "  cloud,"  com- 
posed of  minute  drops  or  dust  floating  in  the  lower  re- 
gions of  the  solar  atmosphere. 

*  The  blackness  of  the  lines  formed  in  this  way  is  such  that  it  is  some- 
times difficult  to  believe,  what  is  really  the  fact,  that  they  are  actually 
brighter  than  they  were  before  the  lime-cylinder  was  placed  behind  the 
flame,  and  that  their  darkness  is  only  apparent,  and  due  to  their  contrast 
with  the  more  brilliant  background  of  the  continuous  spectrum  of  the 
incandescent  lime.  It  is  very  easy,  however,  to  demonstrate  the  truth  by 
a  simple  experiment..  f  See  Note  B,  page  355. 

7 


82  THE  SUN. 

If,  then,  sodium  is  present  in  the  solar  atmosphere 
between  us  and  the  photosphere,  we  ought  to  find  in 
the  solar  spectrum  those  lines  dark  which  are  bright  in 
the  spectrum  of  sodium -vapor ;  and  we  do.  If  mag 
nesium  is  there,  it  ought  to  manifest  itself  in  the  same 
way,  and  it  does ;  and  similarly  for  all  the  substances 
which  spectrum  analysis  reveals. 

If  this  view  is  correct,  it  follows  also  that  this  atmos- 
phere, containing  in  gaseous  form  the  substances  whose 
presence  is  manifested  by  the  dark  lines  of  the  ordi- 
nary spectrum — the  sun's  reversing  layer,  as  it  is  now 
often  called — would  give  a  spectrum  of  bright  lines  if 
we  could  isolate  its  light  from  that  of  the  photosphere. 
The  observation  is  possible  only  under  peculiar  circum- 
stances. At  a  total  eclipse  of  the  sun,  at  the  moment 
when  the  advancing  moon  has  just  covered  the  sun'a 
djsk,  the  solar  atmosphere  of  course  projects  somewhat 
at  the  point  where  the  last  ray  of  sunlight  has  disap- 
peared. If  the  spectroscope  be  then  adjusted  with  its 
slit  tangent  to  the  sun's  image  at  the  point  of  contact,  a 
most  beautiful  phenomenon  is  seen.  As  the  moon  ad- 
vances, making  narrower  and  narrower  the  remaining 
sickle  of  the  solar  disk,  the  dark  lines  of  the  spectrum 
for  the  most  part  remain  sensibly  unchanged,  though 
becoming  somewhat  more  intense.  A  few,  however, 
begin  to  fade  out,  and  some  even  turn  palely  bright  a 
minute  or  two  before  the  totality  begins.  But  the  mo- 
ment the  sun  is  hidden,  through  the  whole  length  of 
the  spectrum,  in  the  red,  the  green,  the  violet,  the 
bright  lines  flash  out  by  hundreds  and  thousands,  almost 
startlingly ;  as  suddenly  as  stars  from  a  bursting  rocket- 
head,  and  as  evanescent,  for  the  whole  thing  is  over 
within  two  or  three  seconds.  The  layer  seems  to  be 
only  something  under  a  thousand  miles  in  thickness, 
and  the  moon's  motion  covers  it  very  quickly. 


THE  SPECTROSCOPE   AND   THE  SOLAR  SPECTRUM.     83 

The  phenomenon,  though  looked  for  at  the  first 
eclipses  after  solar  spectroscopy  began  to  be  a  science, 
was  missed  in  1868  and  1869,  as  the  requisite  adjust- 
ments are  delicate,  and  was  first  actually  observed  only 
in  1870.  Since  then  it  has  been  more  or  less  perfectly 
seen  at  every  eclipse.*  Except  at  an  eclipse  it  has  not 
yet  been  found  possible  to  observe  this  bright-line  spec- 
trum, because  it  is  overpowered  by  the  aerial  illumina- 
tion of  our  own  atmosphere. 

It  is  not,  however,  to  be  understood  that  the  dark 
lines  of  the  solar  spectrum  are  due  entirely  or  even 
principally  to  the  stratum  of  gas  which  lies  close  above 
the  surface  of  the  photosphere.  "Were  this  so,  the 
dark  lines  should  be  much  stronger  in  the  spectrum  of 
light  from  the  edges  of  the  disk  than  in  that  from  the 
center,  which  is  not  the  case  ;  at  least,  the  difference  is 
very  slight.  The  photosphere,  as  we  shall  see  here- 
after, is  probably  composed  of  separate  cloud-like  masses 
floating  in  an  atmosphere  containing  the  vapors  by 
whose  condensation  they  are  formed ;  the  principal  ab- 
sorption, therefore,  probably  takes  place  in  the  inter- 
stices between  the  clouds,  and  below  the  general  level 
of  their  upper  limit. 

The  beautiful  observations  of  Professor  Hastings,  of 
New  Haven,  in  which  by  an  ingenious  contrivance  he 
managed  to  confront  and  compare  directly  the  spectra 
of  light  from  the  center  and  edges  of  the  sun's  disk, 
have  brought  out  the  facts  in  the  case  very  finely. 

Theoretically,  then,  it  is  very  easy  to  test  the  ques- 
tion of  the  presence  of  an  element  in  the  sun.  It  is 
only  necessary  to  cover  one  half  the  length  of  the  spec- 
troscope-slit with  a  mirror  or  prism  by  which  the  sun- 
light is  directed  into  the  instrument,  while  at  the  same 
time  a  flame  or  electric  spark,  giving  the  spectrum  of 
the  substance  under  investigation,  is  placed  directly  in 

*  See  Note  Cv  page  357. 


THE   SUN. 


front  of  the  other  half  of  the  slit.  When  matters  are 
thus  arranged,  the  observer  sees  in  the  instrument  two 
spectra  in  juxtaposition,  each  of  half  the  usual  width — 
one  the  solar  spectrum,  the  other  that  of  the  element 
under  investigation ;  and  it  is  easy  to  see  whether  the 
bright  lines  of  the  elementary  vapor  match  exactly 
with  corresponding  dark  lines  in  the  solar  spectrum. 


FIG.  24. 


FIG.  25. 


ACTION  OF  THE  COMPAKIBON- 
PBISM. 


COMPARISON  PKISM  AT  THE  SLIT  OF  THE  SPBO- 

TBOSCOPE. 


The  figures  show  the  usual  arrangement  of  the  com- 
parison-prism, as  it  is  ordinarily  called. 

For  the  examination  of  the  upper  or  violet  portion 
of  the  spectrum,  photography  is  employed  with  great 
advantage,  the  arrangement  being  precisely  the  same  as 
that  just  indicated,  except  that  a  sensitized  plate  takes 
the  place  of  the  human  retina,  and  the  impression  can 
be  permanently  retained  for  leisurely  study.  Certain 
light,  too,  as  every  one  knows,  which  is  invisible  to  the 
eye,  strongly  affects  the  photographic  plate,  so  that  the 
comparison  can  by  this  means  be  carried  on  into  the 
ultra-violet  regions  of  the  spectrum. 

The  following  full-page  illustration  is  a  representa- 
tion of  the  arrangement  of  apparatus  used  by  Mr.  Lock- 
yer  in  his  celebrated  researches — it  is  taken  from  his 
"  Studies  in  Spectrum  Analysis." 


THE  SPECTROSCOPE   AND   THE   SOLAR  SPECTRUM.      85 


86  THE  SUN. 

Theoretically,  we  say,  the  comparison  is  easy ;  but 
the  practical  difficulties  are  considerable.  In  the  first 
place,  it  is  not  easy  to  get  a  spectrum  of  the  body  you 
wish  to  study,  free  from  lines  belonging  to  other  sub- 
stances— the  requisite  chemical  purity  is  very  trouble- 
some to  attain ;  and,  in  the  next  place,  the  dark  lines  of 
the  solar  spectrum  are  so  numerous  that  it  requires  a 
very  high  dispersive  power  to  establish  a  coincidence 
with  certainty ;  a  bright  line  in  the  spark-spectrum  may 
fall  very  near  a  dark  line  with  which  it  has  no  connec- 
tion whatever.  When,  however,  as  in  the  case  we  have 
mentioned,  the  coincidences  are  not  one  or  two,  but 
numerous,  and  the  lines  in  question  peculiar  in  their 
character  and  appearance,  a  satisfactory  result  is  soon 
established. 

It  was  in  this  manner  (by  comparisons  made  by  the 
eye  and  not  by  photography)  that  Kirchhoff  in  1860  de- 
termined the  presence  in  the  solar  atmosphere  of  the 
following  elements  :  sodium,  iron,  calcium,  magnesium, 
nickel,  barium,  copper,  and  zinc,  the  last  two  rather 
doubtful  at  that  time.  Since  then  the  list  has  been 
greatly  extended,  and  in  1891  stood  as  follows,  accord- 
ing to  Professor  Rowland,  who  has  been  making  a  most 
thorough  reinvestigation  of  the  whole  subject.  He  has 
worked  with  concave-grating  apparatus  of  the  highest 
power,  and  has  made  his  comparisons  between  the  spec- 
trum of  the  sun  and  the  spectra  of  the  chemical  ele- 
ments by  means  of  photography.  He  has  used,  however, 
only  the  electric  arc,  and  not  the  spark,  in  producing 
the  spectra,  and  it  is  to  be  hoped  that  this  research 
may  be  supplemented  by  an  equally  thorough  study  of 
spark-spectra.  Since  the  work  is  not  even  yet  com- 
plete (1895),  the  list  is  to  be  regarded  as  provisional 
only. 


THE   SPECTROSCOPE   AND   THE   SOLAR  SPECTRUM.      87 


ELEMENTS    IN   THE    SUN,    ARRANGED   ACCORDING   TO   THE 

NUMBER    AND    INTENSITY    OF    THEIR    DARK 

LINES    IN   THE    SOLAR    SPECTRUM. 


Intensity. 

1.  Calcium. 

2.  Iron 

3.  Hydrogen. 

4.  Sodium. 

5.  Nickel. 

6.  Magnesium. 

7.  Cobalt. 

8.  Silicon.f 

9.  Aluminium,  f 

10.  Titanium. 

11.  Chromium. 

12.  Strontium. 

13.  Manganese. 

14.  Vanadium. 

15.  Barium. 

16.  Carbon. 

17.  Scandium,  f 

18.  Yttrium. 

19.  Zirconium,  f 

20.  Molybdenum.f 

21.  Lanthanum. 

22.  Niobium,  f 

23.  Palladium.! 

24.  Neodymium.f  ? 

25.  Copper,  f 

26.  Zinc. 

27.  Cadmium. 

28.  Cerium. 

29.  Glucinum.f 

30.  Germanium.f 

31.  -   Rhodium.f 

32.  Silver. 

33.  Tin. 

34.  Lead. 

35.  Erbium. 

36.  Potassium,  f 


Number. 

Iron  (2,000  or  more). 
Nickel. 
Titanium. 
Manganese. 
Chromium. 
Cobalt. 

Carbon  (200  or  more). 
Vanadium. 
Zirconium. 
Cerium. 

Calcium  (75  or  more). 
Neodymium. 
Scandium. 
Lanthanum. 
Yttrium. 
Niobium. 
Molybdenum. 
Palladium. 

Magnesium  (20  or  more). 
Sodium  (11). 
Silicon. 
Hydrogen. 
Strontium. 
Barium. 
Aluminium  (4). 
Cadmium. 
Rhodium. 
Erbium. 
Zinc. 

Copper  (2). 
Silver. 
Glucinum. 
Germanium. 
Tin. 

Lead  (1). 
Potassium. 


88  THE  SUN. 

DOUBTFUL    ELEMENTS. 

Indium.  Osmium.  Platinum.  Ruthenium. 

Tantalum.  Thorium.  Tungsten.  Uranium. 

ELEMENTS  NOT   APPEARING    IN   THE    SOLAR    SPECTRUM. 
Antimony.  Arsenic.  Bismuth.  Boron. 

Caesium.  Gold.  Indium.  Lithium. 

Phosphorus.  Rubidium.  Selenium.  Mercury. 

Thallium.  Praseodymium.     Nitrogen  (vacuum-tube).  Sulphur. 

SUBSTANCES    NOT   YET   TRIED  (BY   ROWLAND). 

Bromine.  Chlorine.  Fluorine.  Iodine. 

Oxygen.  Gallium.  Holmium.  Tellurium. 

Terbium.  Thulium,  etc. 

Professor  Rowland  remarks  that  several  of  the  ele- 
ments are  classified  as  "  not  appearing  in  the  solar  spec- 
trum "  merely  because  their  arc-spectra  show  very  few 
strong  lines,  or  none  at  all,  within  the  limits  of  the  solar 
spectrum  (it  might  be  different  with  their  spark-spec- 
tra). 

He  adds,  what  can  not  too  firmly  be  borne  in  mind, 
that  the  failure  to  find  them  "  is  very  little  evidence  of 
their  absence  from  the  sun  itself,"  and  that  if  the  whole 
earth  "  were  heated  to  the  temperature  of  the  sun,  its 
spectrum  would  probably  resemble  that  of  the  sun  very 
closely." 

Besides  these  substances  which  reveal  their  presence 
in  the  sun  by  dark  lines  in  its  spectrum,  there  are  at 
least  two  others,  helium  *  and  coronium,  as  they  are 
provisionally  named,  which  show  themselves  only  by 
bright  lines  in  the  spectrum  of  the  chromosphere  and 
the  corona,  with  which  we  shall  deal  later.  In  1895 
heliu*m  was  at  last  identified  by  Ramsay,  in  connection 
with  his  researches  upon  argon,  the  new  component  of 
our -atmosphere.  He  found  the  lines  of  helium  in  gas 
*  See  note  on  page  344. 


•  THE   SPECTROSCOPE  AND   THE   SOLAR  SPECTRUM.      89 

disengaged  from  uraninite  and  other  minerals,  where 
it  is  associated  with  the  so-called  u  rare  earths."  Coro- 
nium  still  remains  undetermined. 

All  of  the  above-named  elements,  except  those 
marked  with  a  f,  are  represented  at  times  by  bright  lines 
in  the  spectrum  of  the  chromosphere,  which  will  be  dis- 
cussed in  another  chapter ;  and  strontium  and  cerium 
were  observed  in  that  manner  by  the  writer  before  the 
coincidence  of  their  lines  with  dark,  lines  in  the  ordinary 
solar  spectrum  had  been  satisfactorily  made  out. 

As  to  carbon*  the  characteristic  groups  of  lines 
which  mark  the  visible  portion  of  its  spectrum  are  only 
doubtfully  present;  but  in  the  ultra-violet  the  photo-" 
graphs  of  Mr.  Lockyer  have  brought  out  other  groups 
which  belong  to  this  element,  and  the  presence  of  this 
element  has  since  been  abundantly  confirmed  by  Row- 
land and  others. 

Thus  far  the  most  careful  observation  fails  to  find, 
either  in  the  ordinary  spectrum  or  in  that  of  the  chro- 
mosphere, the  slightest  trace  of  bromine,  chlorine, 
iodine,  or  nitrogen,  of  arsenic,  boron,  or  phosphorus ; 
of  sulphur  there  are  merely  doubtful  indications  in  the 
chromosphere  spectrum;  and  as  regards  oxygen* the 
evidence,  on  the  whole,  is  against  its  presence,  though 
the  case  is  peculiar. 

When  we  recollect  that  the  non-apparent  elements 
constitute  a  great  portion  of  the  earth's  crust,  the 
question  at  once  forces  itself,  What  is  the  meaning  of 
their  seeming  absence  ?  Do  they  really  not  exist  on 
the  sun,  or  do  they  simply  fail  to  show  themselves ;  and, 
if  so,  why  ?  The  answer  to  the  question  is  not  easy, 
and  astronomers  are  not  agreed  upon  it,  though  we  im- 
agine that  most  of  them  would  prefer  the  latter  alterna- 
tive. Even  under  the  conditions  of  our  terrestrial 
*  See  note  on  page  350. 


90  THE   SUN. 

laboratories  we  find  cases  where,  when  several  gases  and 
vapors  are  mingled  at  a  high  temperature,  certain  ones 
only  of  those  present  appear  in  the  spectrum  of  the 
mixture,  the  others  giving  no  indication  of  their  pres- 
ence. Then,  too,  it  is  now  certain  that  the  same  sub- 
stance under  differing  conditions  may  give  two  or  more 
widely  differing  spectra ;  it  is  easy  to  admit,  therefore, 
that  we  may  be  unable  to  reproduce  in  the  electric  arc 
the  spectrum  'of  a  substance  that  characterizes  it  in 
the  sun,  and  so  may  fail  to  identify  it.  Possibly,  in 
some  cases,  the  very  brilliance  of  the  lines  of  an  element 
may  prevent  their  appearance  as  dark  lines.  It  is  pos- 
^ible,  for  instance,  to  make  the  bright  lines  of  sodium 
so  intense  that  the  light  from  an  incandescent  lime- 
cylinder  will  not  be  able  to  reverse  them,  and,  of  course, 
by  making  them  a  little  less  intense,  they  may  be  caused 
to  disappear  entirely,  being  neither  brighter  nor  darker 
than  the  continuous  spectrum  on  which  they  are  pro- 
jected. This  may  perhaps  actually  be  the  case  with 
helium,  which  gives  in  the  chromosphere  spectrum  an 
intensely  brilliant  yellow  line,  known  as  D8,  because  it 
is  very  near  to  the  sodium  lines,  D  and  Da.  At  times, 
and  especially  in  the  neighborhood  of  sun-spots,  a  very 
faint  dark  line  marks  its  place,  but  usually  the  spec- 
trum of  the  photosphere  fails  to  give  the  slightest  in- 
dication of  its  presence.  There  are,  however,  in  the 
chromosphere  spectrum  fifteen  or  twenty  other  bright 
lines  (but  fainter  than  D8)  which  have  no  dark  cor- 
relatives. Most  of  these  are  now  known  to  be  also 
due  to  helium,  and  this  makes  it  more  likely  that  the 
absence  of  dark  lines  is  accounted  for  either  by  the 
thinness  of  the  helium  layer  or  the  intensity  of  its  tem- 
perature. 

Nitrogen  and  hydrogen  each  have  two  spectra,  one 


THE   SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.     91 

a  spectrum  mostly  composed  of  shaded  bands,  while  the 
other  consists  of  sharp,  well-defined  lines.  Oxygen,  ac- 
cording to  Schuster's  careful  researches,  has  four  spec- 
tra, and  carbon  is  also  assigned  four  by  its  investigators. 
There  appear  to  be  various  possible  explanations  of 
these  facts.  One  is,  to  suppose  that  the  luminous  sub- 
stance, without  any  change  in  its  own  constitution,  vi- 
brates differently  and  emits  different  rays  under  varying 
circumstances,  just  as  a  metal  plate  emits  various  notes 
according  to  the  manner  in  which  it  is  held  and  struck. 
A  second  assumes  that  the  substance,  without  losing  its 
chemical  identity,  undergoes  changes  of  molecular  struct- 
ure (assumes  allotropic  forms)  under  the  varying  cir- 
cumstances which  produce  the  changes  in  its  spectrum. 
According  to  either  of  these  views,  although  we  can 
safely  infer,  from  the 'presence  of  the  known  lines  of  an 
element  in  the  solar  spectrum,  its  presence  in  the  solar 
atmosphere,  we  can  not  legitimately  draw  any  negative 
conclusion :  the  substance  may  be  present,  but  in  such 
a  state  under  the  solar  conditions  as  to  give  a  spectrum 
different  from  any  with  which  we  are  acquainted. 

Still  a  different  explanation  is  to  suppose,  with  Mr. 
Lockyer,  that  the  changes  in  the  spectrum  of  a  body  are 
indications  of  its  decomposition,  the  spectrum  of  the 
original  substance  being  replaced  by  the  superposed 
spectra  of  its  constituents,  so  that  the  absence  of  the 
missing  substances  is  real,  being  due  simply  to  the  fact 
that  the  solar  atmosphere  is  too  hot  to  permit  them  to 
exist  in  it :  they  decompose  or  "  dissociate  "  at  a  lower 
temperature. 

It  would  be  improper  to  dismiss  this  hypothesis  with 
a  mere  passing  mention,  for  during  the  past  fifteen  years 
it  has  been  almost  constantly  under  brisk  discussion. 
Whether  true  or  not,  it  is  certainly  not  absurd,  nor  in 


92  THE  SUN. 

itself  even  improbable.  The  idea  that  at  bottom  there 
is  but  one  material  substance,  and  that  all  our  chemical 
elements  differ  only  in  the  way  in  which  their  ultimate 
molecules  are  built  up  out  of  the  simple  atoms  of  this 
"  pantogen,"  is  old,  and  has  always  been  attractive  to 
speculative  minds.  Grant  it,  and  many  otherwise  puz- 
zling facts  and  relations  of  the  new  chemistry  become  in- 
telligible. At  the  same  time  it  has  not  yet  been  proved, 
and  so  far  all  attempts  to  break  up  the  elements  into 
simpler  bodies  have  failed.  It  seems  impossible  also  to 
reconcile  the  hypothesis  with  the  laws  which  connect 
the  specific  heat  of  bodies  with  their  chemical  constitu- 
tion and  atomic  weight. 

It  may  be  added,  too,  that  some  of  the  supposed  ob- 
servational facts  upon  which  Mr.  Lockyer  relied  at 
first  to  support  his  theory  have  turned  out  to  be  mis- 
takes due  to  errors  of  experimentation,  or  the  use  of 
insufficient  spectroscopic  power. 

Thus  great  stress  was  laid  upon  the  so-called  "  basie  " 
lines  which  appear  to  be  common  to  the  spectra  of  dif- 
ferent substances.  If  one  runs  over  Angstrom's  map 
of  the  solar  spectrum  he  will  find  about  twenty-five  lines 
marked  as  belonging  both  to  iron  and  calcium.  The 
same  is  true  of  iron  and  titanium  to  a  still  greater 
extent,  and  to  a  considerable  degree  of  several  other 
pairs  of  substances.  This  fact  might  be  explained  in 
several  ways.  The  common  lines  may  be  due — first,  to 
impurities  in  the  materials  worked  with ;  or,  second,  to 
some  common  constituent  in  the  substances  (which  is 
Lockyer's  view) ;  or,  third,  to  some  similarity  of  molec- 
ular mass  or  structure  which  determines  an  identical 
vibration-period  for  the  two  substances ;  or,  finally,  it 
may  be  that  the  supposed  coincidence  of  the  lines  is 
only  apparent  and  approximate — not  real  and  exact — 


THE   SPECTROSCOPE   AND   THE   SOLAR   SPECTRUM.      93 

in  which  case  a  spectroscope  of  sufficient  dispersive 
power  would  show  the  want  of  coincidence. 

Now,  Mr.  Lockyer,  by  a  series  of  most  laborious 
researches,  has  proved  that  many  of  the  coincidences 
shown  on  the  map  are  merely  due  to  impurities,  and 
he  has  been  able  to  point  out  which  of  the  lines  mapped 
as  common  to  calcium  and  iron,  for  instance,  belonged 
to  each  metal.  As  the  iron  employed  is  rendered  suc- 
cessively purer  and  purer,  certain  of  the  common  lines 
become  fainter,  and  such  evidently  belong  to  calcium 
and  not  to  iron.  Similarly,  when  calcium  is  used,  we 
can  point  out  the  lines  which  are  due  to  the  iron  con- 
tamination. But,  when  all  is  done,  we  find  that  certain 
of  the  common  lines  persist,  becoming  more  and  more 
conspicuous  with  every  added  precaution  taken  to  in- 
sure purity  of  materials. 

Moreover,  when  one  of  the  substances,  say  the  cal- 
cium, is  subjected  to  continually  increasing  tempera- 
tures, its  spectrum  is  continually  modified,  and  these 
basic-lines,  as  Mr.  Lockyer  asserts,  are  the  ones  which 
become  increasingly  conspicuous,  while  others  disappear. 
This  is  just  what  ought  to  happen  if  they  are  due  to 
some  element  existing  in  both  the  iron  and  calcium — an 
element  liberated  in  increasing  abundance  with  every 
rise  of  temperature. 

But  unfortunately  for  the  theory  the  application  of 
our  present  powerful  spectroscopes  shows  that  in  almost 
every  case  these  "  basic "  lines  are  only  instances  of 
close  coincidence.  The  writer  in  1880  examined  with 
care  the  seventy  lines  given  on  Angstrom's  map  as  com- 
mon to  two  or  more  elements,  and  was  able  to  resolve 
fifty-six  of  them  into  doubles  or  triplets ;  and  later  ob- 
servers have  resolved  the  rest  or  shown  that  they  were 
due  to  impurities.  Professor  Rowland  remarks  that 


94:  THE  SUN. 

"  with  the  high  dispersion  "  employed  by  him  "  Lock- 
yer's  '  basic-lines '  are  widely  broken  up  and  cease  to 
exist." 

As  has  been  already  remarked,  the  case  of  oxygen  is 
peculiar.  The  great  A  and  B  bands  of  the  solar  spec- 
trum are  certainly  due  to  this  gas,  as  Egoroff  first  defi- 
nitely proved  in  1883  ;  but,  as  the  experiments  and  ob- 
servations of  Janssen  and  others  have  since  abundantly 
demonstrated,  it  is  the  oxygen  of  the  earth's  atmosphere 
and  not  oxygen  in  the  sun  that  produces  them ;  they 
belong  to  a  low-temperature  spectrum  of  the  gas,  and 
not  to  the  spectrum  produced  by  the  electric  arc  or 
spark. 

But  in  1877  the  late  Dr.  Henry  Draper,  of  New 
York,  announced  that  he  had  discovered  the  presence  of 
oxygen  in  the  sun,  and  he  published  photographs  which 
show,  in  a  very  plausible  manner,  the  coincidence  be- 
tween the  bright  lines  of  this  element  and  certain  bright 
spaces  or  bands  in  the  solar  spectrum.  His  method  of 
precedure  was  to  form  the  spectrum  of  oxygen  by 
means  of  sparks  from  a  powerful  induction-coil,  worked 
by  a  dynamo-electric  machine,  itself  driven  by  an  en- 
gine. These  sparks  passed  between  iron  terminals,  in 
a  little  chamber  wrought  out  of  soapstone,  through  which 
a  current  of  pure  oxygen  was  forced  at  atmospheric 
pressure  nearly ;  sometimes,  however,  air  was  used  in- 
stead, giving  the  same  results,  except  that  the  spectrum 
of  nitrogen  was  then  superadded  to  that  of  oxygen. 
The  spectrum  of  this  spark  was  photographed  simultane- 
ously with  that  of  the  sun,  the  sunlight  being  brought 
in  through  half  the  slit  by  a  small  reflector,  and  thus  a 
comparison  was  obtained,  free  from  personal  bias,  be- 
tween the  solar  spectrum  and  that  of  the  gas.  The  iron 
lines,  due  to  the  terminals,  are  a  great  assistance  in  test- 


THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.      95 

ing  the  adjustments.  The  oxygen  lines  produced  in  this 
way  at  atmospheric  pressure  are  not  so  well  defined  as 
those  seen  in  the  spectrum  of  a  Geissler  tube,  but  are 
rather  broad  and  hazy. 

In  the  blue  portion  of  the  solar  spectrum,  which 
alone  is  accessible  to  photography,  the  Fraunhofer  lines 
are  generally  very  numerous,  close,  and  black ;  but  here 
and  there  is  an  interval  free,  or  comparatively  free, 
from  lines.  In  a  low-dispersion  spectroscope  such  an 
interval  looks  like  a  bright  band.  Now,  almost  every 
one  of  the  dozen  or  so  bright  lines  of  oxygen,  which 
the  photographs  display,  falls  exactly  against  one  of 
these  brighter  interspaces,  and  it  seems  hardly  probable 
that  so  many  coincidences  can  be  merely  due  to  chance. 

Dr.  Draper  afterward  repeated  the  laborious  and 
expensive  experiments  in  a  still  more  elaborate  manner, 
and  with  results  entirely  confirmatory. 

It  is,  however,  extremely  difficult  to  explain  how 
oxygen  in  the  sun's  atmosphere  can  produce  such  an 
effect  in  the  ordinary  solar  spectrum  while  remaining 
invisible  in  the  spectrum  of  the  chromosphere  ;  and  the 
most  careful  search  does  not  show  in  it  a  single  one  of 
these  bright  oxygen-lines.  We  say  of  these  lines,  because 
Dr.  Schuster  has  shown,  with  great  probability,  that  a 
different  oxygen  spectrum,  with  only  four  bright  lines 
in  it,  has  these  four  all  represented  by  dark  lines  in  the 
photospheric  spectrum,  and  two  of  the  four  in  the  spec- 
trum of  the  chromosphere. 

With  high  dispersive  powers,  the  "bright  bands" 
of  the  solar  spectrum  entirely  lose  their  prominence,  and 
are  even  found  to  be  occupied  by  numerous  fine  dark 
lines.  Dr.  John  C.  Draper  has  suggested  that  these 
dark  lines  may  be  the  true  representatives  of  oxygen. 

Still  later  photographic  comparisons  between   the 


96  THE   SUN. 

solar  spectrum  and  that  of  oxygen,  made  with  high  dis- 
persive powers,  in  the  Physical  Laboratory  of  Harvard 
College  and  at  other  places,  render  it  certain  that  no 
coincidences  of  solar-lines  with  the  bright  lines  in  the 
line-spectrum  of  oxygen  exist  in  the  region  covered  by 
the  photographic  plates  that  were  used  (\  3,750  to 
X  5,034).  ,  Probably  most  spectroscopists  consider  this 
conclusive  as  to  the  absence  of  oxygen  from  the  solar 
spectrum  :  at  the  same  time,  as  Professor  Pickering  has 
shrewdly  said,  "  one  would  scarcely  expect  to  recognize 
a  friend's  countenance  under  the  microscope.'1''  The 
discussion  can  hardly  be  considered  as  finally  closed. 

The  lines  of  the  solar  spectrum  not  only  inform  us 
as  to  the  presence  or  absence  of  bodies  in  the  solar 
atmosphere,  but  give  us,  to  some  extent,  indications  as 
to  their  physical  condition.  The  spectrum  of  a  given 
body,  say  hydrogen,  varies  very  much  in  the  relative 
strength  and  brightness  of  its  lines,  according  to  the 
circumstances  of  its  production.  If,  for  instance,  the 
gas  be  highly  rarefied,  and  the  electric  spark,  which 
illuminates  it,  not  too  strong,  the  lines  will  be  fine  and 
sharp.  Under  higher  pressure  and  more  intense  dis- 
charges, some  of  them  will  become  broad  and  hazy,  and 
new  lines,  before  unseen,  will  make  their  appearance. 
So  of  other  substances ;  and  this  apart  from  the  fact, 
before  stated,  that  a  given  element  often  has  several  en- 
tirely different  spectra.  Changes,  such  as  have  been 
mentioned,  go  on  up  to  a  certain  point,  and  then,  sud- 
denly, an  entirely  new  spectrum  appears,  not  having 
apparently  the  slightest  connection  with  the  one  which 
preceded  it  any  more  than  if  it  came  from  an  entirely 
different  element  or  mixture  of  elements ;  as,  in  fact, 
according  to  Mr.  Lockyer's  view,  is  probably  the  case. 

Now,  in  the  solar  spectrum,  the  dark  lines  character- 


THE   SPECTROSCOPE   AND   THE   SOLAR   SPECTRUM.      97 

istic  of  an  element  are  all  coincident  with  bright  lines 
of  its  gaseous  spectrum  ;  but  it  is  often  the  case  that 
the  relative  width  and  intensity  of  the  solar-lines  do  not 
match  those  of  the  bright  lines  in  the  spectrum  ob- 
tained by  artificial  means.  In  the  spectrum  of  calcium, 
for  instance,  certain  lines,  which  in  our  laboratory  ex- 
periments are  the  most  conspicuous,  are  very  faint  upon 
the  sun,  and  others,  which  are  inconspicuous  in  the  spark 
spectrum,  are  vastly  more  important  on  the  solar  sur- 
face. As  yet,  we  are  not  able  with  certainty  to  inter- 
pret all  these  variations,  but,  in  a  general  way,  it  may  be 
said  that  they  all  point  to  the  conclusion  that  the  tem- 
perature of  the  solar  atmosphere  is  considerably  higher 
than  that  of  any  of  our  flames  or  electric  arcs. 

SPECTKOSCOPIC    INDICATIONS    OF   MOTION. 

At  times,  also,  when  the  motions  of  the  solar  atmos- 
phere become  unusually  intense,  the  spectroscope  ap- 
prises us  of  the  fact,  and  gives  us  the  means  of  deter- 
mining the  rate  at  which  the  moving  masses  are  advanc- 
ing toward  us  or  receding  from  us.  If  a  luminous 
body  is  approaching  with  a  velocity  at  all  comparable 
with  that  of  light,  the  pitch  of  the  light,  if  the  expres- 
sion may  be  allowed — its  wave-length  and  number  of 
vibrations  per  second — will  be  changed  and  heightened 
just  as  in  the  case  of  sound. 

Most  of  our  readers  have  probably  noticed  the  curi- 
ous change  in  pitch  of  the  bell  or  whistle  of  a  locomo- 
tive passing  at  full  speed,  especially  if  we  ourselves 
were  on  a  train  moving  in  the  opposite  direction.  If 
the  velocity  is  great  (about  forty  miles  an  hour  for  each 
of  the  trains)  the  pitch  will  drop  a  full  third. 

The  explanation,  first  given  by  Doppler,  of  Prague, 
in  1842,  is  simply  this :  If  both  we  and  the  locomotive 
8 


98  THE  SUN. 

carrying  the  bell  were  at  rest,  we  should  hear  the  bell's 
true  sound,  the  pulsations  following  each  other  at  regu- 
lar and  the  real  intervals.  If,  now,  we  are  rapidly  ap- 
proaching the  bell,  the  interval  of  time  between  the 
impact  of  each  pulse  upon  the  ear  and  the  following 
one  will  be  shortened,  because  after  any  pulse  has  been 
received  we  advance  part  way  to  meet  the  next,  and  so 
encounter  it  earlier  than  if  we  had  remained  at  rest. 
Now,  this  interval  of  time  between  successive  pulsations 
is  precisely  what  determines  the  pitch  of  the  sound  :  the 
more  pulsations  there  are  in  a  second  the  higher  the 
pitch.  It  is  obvious  that,  if  we  remain  at  rest  and  the 
bell  approaches  us,  the  same  effect  will  be  produced, 
and  that,  if  both  are  moving,  the  effects  will  be  added ; 
and,  finally,  it  is  clear  that  the  recession  of  the  hearer 
from  the  bell  will  produce  the  opposite  effect  and  lower 
its  pitch. 

Just  the  same  thing  holds  good  of  light ;  it  also  con- 
sists of  pulsations,  and  the  refrangibility  of  a  ray  and 
its  diffrangibility,  if  we  may  coin  the  word,  both  de- 
pend upon  the  number  of  pulsations  per  second  with 
which  it  reaches  the  diffracting  or  refracting  surface. 
The  more  frequent  the  pulsations  the  more  it  will  be 
refracted,  and  the  less  it  will  be  diffracted.  If,  then, 
we  were  swiftly  approaching  a  mass,  say  of  incandes- 
cent hydrogen,  we  should  find  the  position  of  each  of 
its  characteristic  rays  in  the  spectrum  slightly  altered, 
and  falling  farther  from  the  red  end  of  the  spectrum 
(the  region  of  slow  vibrations)  than  if  we  were  at  rest. 
By  comparing  the  positions  of  these  lines  with  those 
obtained  from  a  Geissler  tube  containing  hydrogen,  we 
could  find  how  much  change  was  produced,  and  there- 
fore how  the  velocity  with  which  we  are  approaching 
the  moving  mass  compares  with  that  of  light.*  Simi- 
*  See  Note  D,  page  359. 


THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.      99 

larly,  if  the  body  were  advancing  toward  us.  And,  vice 
versa,  if  the  distance  were  increasing,  the  lines  would 
be  shifted  downward  in  the  spectrum  toward  the  red.* 

Because  the  velocity  of  light  is  exceedingly  great 
(more  than  186,000  miles  per  second),  it  is  evident  that 
only  very  swift  motions  can  produce  any  sensible  dis- 
placement of  lines  in  the  spectrum.  Since,  however, 
in  the  neighborhood  of  sun-spots  and  in  the  solar  prom- 
inences, we  frequently  meet  with  masses  of  gas  moving 
from  thirty  to  fifty  miles  a  second,  and  sometimes  as 
much  as  three  hundred  miles  a  second,  it  is  not  unusual, 
in  working  with  the  telespectroscope,  to  observe  the  dis- 
tortion and  displacement  of  portions  of  a  dark  line 
which  are  produced  by  these  motions,  and  indicate 
them. 

The  figure  represents  the  appearance  of  the  C  line 
seen  in  the  spectrum  of  a  sun-spot  by  the  writer  on 
Sept.  22,  1870.  The  velocities  indicated  vary  from  two 
hundred  and  thirty  to  three  hundred  and  twenty  miles 
per  second  ;  the  latter  is  seldom,  if  ever,  exceeded. 

Results  of  this  sort  are  so  surprising  that  there  have 
been  many  attempts  to  escape  from  them,  and  to  ac- 
count for  the  distortion  of  lines  in  some  other  way,  but 

*  The  formula  for  computing  the  change  of  wave-length  produced  by  a 
given  velocity  along  the  line  of  sight  is  very  simple.  Let  A  be  the  real  wave- 
length of  the  ray  ;  A',  the  apparent  wave-length  as  affected  by  the  motion ; 
F,  the  velocity  of  light  (186,330  miles  a  second) ;  and  v,  the  rate  at  which 
the  distance  between  the  observer  and  the  source  of  light  is  increasing, 


then  A'  —  A  =  A— ,  which  may.  be  written  A  A  =  A.—.     If  the  distance  is 

decreasing r,  v  must  be  taken  as  minus,  and  A.'  will  be  less  than  A. 

As  an  example,  suppose  that  near  a  sun-spot  a  mass  of  hydrogen  is 
approaching  us  at  the  rate  of  50  miles  a  second :  how  much  will  the  wave- 
length of  the  C  line  (\  =  6,563  units)  be  diminished  ? 

A  A  =  6,563  x  TTtH?w  =  fM  =  1-V7  units.  That  is,  the  C  line  will 
be  shifted  up  (toward  the  blue)  1'77  units  on  the  scale  of  Rowland's  map. 


100 


THE  SUN. 


without  any  satisfactory  success.  There  have  been  diffi- 
culties raised  also  in  regard  to  the  mathematical  theory 
of  the  matter.  These  have  been  met,  however;  and 
what  amounts  to  an  experimental  verification  of  the 
correctness  of  the  received  view  has  been  reached  by 
measurements  of  the  displacement  of  lines  in  the  spec- 
tra of  the  eastern  and  western  limbs  of  the  sun.  The 
eastern  limb  is  moving  toward  us,  the  western  from  us, 


FIG.  27. 


CHANGES  IN  THH  C  LINE  (September  22,  1870). 

in  consequence  of  the  sun's  rotation,  each  with  a  ve- 
locity of  about  1'16  miles  per  second.  The  resulting 
displacement  of  the  lines  is,  of  course,  very  slight — only 
about  YTO"  °^  the  distance  between  the  two  D  lines — 
but,  small  as  it  is,  it  has  been  satisfactorily  detected  and 
measured  by  several  observers — Zolliier,  Yogel,  Lang- 
ley,  and  the  writer,  among  the  earlier. 

The  values  determined  have  ranged  generally  some- 
what larger  than  1*16.  My  own  result  was  1'42  ±  0*07, 
and  was  obtained  in  1876  with  the  first  grating-spectro- 
scope used  in  astronomical  work. 

A  later  determination  made  by  Crew,  at  Baltimore, 
with  a  much  more  powerful  instrument,  gave  1*18.  The 


THE  SPECTROSCOPE  AND  THE  SOLAR  SPECTRUM.  1Q1 

most  complete  and  satisfactory  series  of  observations  of 
this  sort  is,  however,  that  made  by  Duner  in  1887-' 89, 
which  not  only  gave  a  good  determination  of  the 
sun's  rotation  period  (25 '56  days),  agreeing  closely  with 
that  deduced  from  spot  observations,  but  also  brought 
out  clearly  the  "  equatorial  acceleration  "  (page  140). 

Cornu  has  made  a  beautiful  application  of  this  prin- 
ciple to  enable  one  to  discriminate  immediately  between 
the  lines  in  the  solar  spectrum  which  are  really  "  solar  " 
and  the  "  telluric  "  lines,  which  are  due  to  our  own  at- 
mosphere. A  small  image  of  the  sun  is  formed  upon 
the  slit-plate  of  the  spectroscope  by  a  lens  which  can  be 
made  to  swing  back  and  forth  three  or  four  times  a 
second.  This  makes  the  solar  image  oscillate  across  the 
slit-plate  in  an  east  and  west  direction,  and  to  the  ob- 
server all  the  true  solar  lines  appear  to  quiver,  while 
the  telluric  lines  stand  fast. 

In  the  motion-distortions  of  lines  Lockyer  finds 
strong  confirmation  of  his  ideas.  It  not  unfrequently 
happens  that  in  the  neighborhood  of  a  spot  certain  of 
the  lines  which  we  recognize  as  belonging  to  the  spec- 
trum of  iron  give  evidence  of  violent  motion,  while, 
close  to  them,  other  lines,  equally  characteristic  of  the 
laboratory  spectrum  of  iron,  show  no  disturbance  at  all. 
If  we  admit  that  what  we  call  the  spectrum  of  iron  is 
really  formed  in  our  experiments  by  the  superposition 
of  two  or  more  spectra  belonging  to  its  constituents, 
and  that  on  the  sun  these  constituents  are  for  the  most 
part  restricted  to  different  regions  of  widely  varying 
pressure,  temperature,  and  elevation,  it  becomes  easy  to 
see  how  one  set  of  the  lines  may  be  affected  without  the 
other. 

But  the  same  facts  are  also  explicable  on  various 
other  hypotheses. 


CHAPTEE  IV. 

SUIT-SPOTS  AND   THE  SOLAR  SURFACE. 

Granulation  of  Solar  Surface. — Views  of  Langley,  Nasmith,  Secchi,  and 
others. — Faculae. — Nature  of  the  Photosphere. — Janssen's  Photo- 
graphs of  Solar  Surface — the  Reseau  Photospherique. — Discovery  of 
Sun-Spots. — General  Appearance  and  Structure  of  a  Spot. — Its  For- 
mation and  Disappearance. — Duration  of  Sun-Spots. — Remarkable 
Phenomena  observed  by  Carrington  and  Hodgson. — Observations  of 
Peters. — Dimensions  of  Spots. — Proof  that  Spots  are  Cavities. — Sim- 
Spot  Spectrum.—"  Veiled  Spots."— Rotation  of  Sun.— Equatorial  Ac- 
celeration.— Explanations  of  the  Acceleration. — Position  of  Sun's 
Axis  and  Secchi's  Table  for  its  Position-Angle  at  Different  Times  of 
the  Year. — Proper  Motions  of  Spots. — Distribution  of  Spots. 

WHEN  an  observer,  provided  with  suitable  telescopic 
appliances,  examines  the  surface  of  the  sun,  he  finds  a 
most  interesting  field  before  him.  At  first  view,  in- 
deed, it  is  less  impressive  than  the  moon ;  there  is  not 
so  much  to  attract  the  immediate  attention — no  moun- 
tain-ranges and  craters,  no  shadows,  rills,  or  rays. 

But,  if  the  telescope  is  a  good  one  and  the  atmos- 
pheric conditions  favorable,  the  details  soon  begin  to 
come  out :  the  surface  is  seen  to  be  far  from  uniform, 
composed  of  minute  grains  of  intense  brilliance  and  ir- 
regular form,  floating  in  a  darker  medium,  and  arranged 
in  streaks  and  groups.  If  the  magnifying  power  em- 
ployed is  rather  low,  the  general  effect  of  the  surface  is 
much  like  that  of  rough  drawing-paper,  or  of  curdled 
milk  seen  from  a  little  distance ;  and,  generally  speak- 
ing, a  low  power  is  all  that  can  be  used,  because  the 


SUN-SPOTS  AND  THE  SOLAR  SURFACE.  1Q3 

heat  of  the  sun  commonly  keeps  the  air  in  a  state  of  great 
disturbance,  so  that  it  is  only  occasionally  that  the  solar 
surface  can  be  scrutinized  with  such  powers  as  we  con- 
tinually employ  upon  the  moon  and  planets.  But  now 
and  then  times  come — favorable  minutes,  and  even  hours 
— when  the  telescopic  power  can  be  pushed  to  its  maxi- 
mum, and  we  get  such  views  as  that  which  Professor 
Langley  has  presented  in  the  beautiful  drawing  of  which 
our  frontispiece  is  a  reproduction.  The  grains,  or  "  nod- 
ules," as  Herschel  called  them,  are  then  seen  to  be  ir- 
regularly rounded  masses,  measuring  some  hundreds  of 
miles  each  way,  sprinkled  upon  a  less  brilliant  back- 
ground, and  making  much  the  same  impression  as  snow- 
flakes  sparsely  scattered  over  a  grayish  cloth,  to  use  the 
comparison  of  Professor  Langley.  If  the  telescope  has 
a  diameter  of  not  less  than  nine  inches,  and  if  the  see- 
ing is  absolutely  exquisite,  then  these  grains  themselves 
are  sometimes  resolved  into  "  granules,"  little  luminous 
dots  not  more  than  a  hundred  miles  or  so  in  diameter, 
which,  by  their  aggregation,  make  up  the  grains,  just  as 
they  in  their  turn  make  up  the  coarser  masses  of  the 
solar  surface.  Professor  Langley  estimates  these  gran- 
ules to  constitute  perhaps  about  one  fifth  of  the  surface 
of  the  sun,  while  they  emit  at  least  three  quarters  of  the 
light.  He  and  Secchi  seem  to  be  so  far  the  only  ob- 
servers who  have  ever  fairly  seen  them.  The  "  grains  " 
have  been  known  for  years  and  described  by  many 
observers,  but  with  some  very  embarrassing  discrep- 
ancies. Nasmyth,  in  1861,  described  them  as  "wil- 
low-leaves "  in  shape,  several  thousand  miles  in  length, 
but  narrow,  with  pointed  ends ;  and  figured  the  surface 
of  the  sun  as  a  sort  of  basket-work  formed  by  the  inter- 
weaving of  such  filaments.  Fig.  28  is  copied  from  one 
of  his  pictures.  His  statement  excited  a  good  deal  of 


104 


THE  SUN. 


Fre.  28. 


GROUP  OF  BOLAB  SPOTS  OBSEBVBD  AND  DRAWN  BY  NASMYTH  (June  5,  1864). 


SUN-SPOTS  AND  THE  SOLAR  SURFACE. 


105 


pretty  warm  discussion.  Dawes  entirely  denied  the 
existence  of  any  such  forms;  while  Stone  and'Secchi 
assigned  them  much  smaller  dimensions,  and  compared 
them  to  rice-grains.  Huggins  agreed  completely  with 


FIG.  29. 


flRANTTLES   AND   PORES   OF  THB   SlJN'8   SURFACE.      (After  HugginS.) 

neither,  but  represents  the  "  make-up  "  of  the  solar  sur- 
face by  a  drawing  from  which  Fig.  29  is  taken.  This 
is  unquestionably  a  very  correct  delineation  of  what  is 
seen  with  a  good  telescope  under  circumstances  fair 
but  not  the  best  possible. 


106  THE  SUN. 

On  portions  of  the  sun's  disk,  however,  the  element- 
ary structure  is  often  composed  of  long,  narrow,  blunt- 
ended  filaments,  not  so  much  like  "  willow-leaves "  as 
like  bits  of  straw,  lying  roughly  parallel  to  each  other 
— a  "thatch-straw"  formation,  as  it  has  been  called. 
This  is  specially  common  in  the  penumbras  of  spots,  or 
in  their  immediate  neighborhood. 

If  one  were  to  speculate  as  to  the  explanation  of  the 
grains  and  thatch-straws,  it  might  be  that  the  grains  are 
the  upper  ends  of  long  filaments  of  luminous  cloud, 
which,  over  most  of  the  sun's  surface,  stand  approxi- 
mately vertical,  but  in  the  penumbra  of  a  spot  are  in- 
clined so  as  to  lie  nearly  horizontal.  This  is  not  certain, 
though ;  it  may  be  that  the  cloud-masses  over  the  more 
quiet  portions  of  the  solar  surface  are  really,  as  they 
seem,  nearly  globular,  while  near  the  spots  they  are 
drawn  out  into  filamentary  forms  by  atmospheric  cur- 
rents. 

Whatever  the  explanation  may  be,  the  appearance 
of  things  in  the  immediate  neighborhood  of  a  spot  is 
often  pretty  fairly  represented  by  Mr.  Nasmyth's  pict- 
ures, though  that  of  Professor  Langley  is  decidedly 
more  accurate  in  details,  and  represents  far  better  see- 
ings. 

Near  the  edges  of  the  disk  the  light  falls  off  very 
rapidly,  and  certain  peculiar  formations,  called  the  fac- 
ulse,  are  there  much  more  noticeable  than  near  the  cen- 
ter of  the  disk.  These  f aculae  (Latin,  "  a  little  torch  ") 
are  irregular  streaks  of  greater  brightness  than  the  gen- 
eral surface,  looking  much  like  the  flecks  of  foam  which 
mark  the  surface  of  a  stream  below  a  waterfall.  Not 
unfrequently  they  are  from  five  to  twenty  thousand 
miles  in  length,  covering  areas  immensely  larger  than 
any  terrestrial  continent. 


SUN-SPOTS  AND   THE  SOLAR  SURFACE.  107 

The  figure,  taken  from  a  photograph  by  De  La  Rue, 
gives  a  reasonably  correct  idea  of  the  general  appear- 
ance of  these  objects,  and  of  the  darkening  at  the  limb 
of  the  sun.  No  woodcut,  however,  is  quite  competent 
to  give  the  delicate  flocculence  of  the  details. 

Until  lately  these  faculse  have  been  considered  to  be 
simply  elevated  portions  of  the  photosphere — mountain- 
ous billows  of  shining  cloud  which  rise  above  the  gen- 

Fio.  80. 


Strx-SpoTS  AND  FACTTI-^E.     (From  a  Photograph.) 

eral  level,  and  protrude  through  the  denser  portions  of 
the  solar  atmosphere.  Occasionally,  when  one  of  them 
passes  over  the  edge  of  the  disk,  it  can  be  seen  to  pro- 
ject like  a  little  tooth — the  reader  should  not  forget, 
however,  that  the  elevation,  to  be  perceptible  at  all, 
must  be  at  least  two  hundred  and  twenty-five  miles,  or 
some  forty-five  times  as  high  as  any  Himalaya. 

If  they  are  elevations  rising  from  the  photosphere, 
the  reason  why  they  are  so  much  more  conspicuous 


108 


THE   SUN. 


near  the  limb  is  simply  this :  The  luminous  surface  is 
covered,  as  has  been  intimated  before,  with  an  atmos- 
phere which  is  not  very  thick  compared  with  the  di- 
mensions of  the  sun,  but  still  sufficient  to  absorb  a  good 
deal  of  the  light.  Light  coming  from  the  center  of  the 
disk  penetrates  this  atmosphere  at  #,  as  is  apparent  from 
the  figure,  under  the  most  favorable  conditions,  and  is 
but  slightly  reduced  in  amount.  The  edges  of  the  disk, 
on  the  other  hand,  are  seen  through  a  much  greater 
thickness  of  atmosphere,  as  at  &,  and  are,  therefore,  of 
course,  much  obscured,  the  amount  of  absorption  being 


FIG.  81. 


by  some  observers  put  as  high  as  seventy-five  per  cent. 
If,  now,  to  take  an  extreme  case,  we  suppose  a  facula 
high  enough  to  lift  its  summit  quite  through  this  at- 
mosphere, it  will  itself  suffer  no  diminution  of  brill- 
iance while  the  sun's  rotation  carries  it  from  the  center 
of  the  disk  to  the  limb,  but  it  will  have  passed  from 
a  background  of  brightness  almost  equal  to  its  own, 
on  which  it  would  be  seen  only  with  difficulty,  to 
another  seventy-five  per  cent,  or  so  darker,  and  will 
thus  become  very  conspicuous.  What  is  true  of  faculse 
of  such  extreme  dimensions  is,  of  course,  also  measura- 
bly true  of  those  of  inferior  elevation.  The  recent  pho- 
to-spectrographic  work  of  Hale  and  Deslandres  suggests, 


SUN-SPOTS  AND  THE  SOLAR  SURFACE.  109 

however,  a  different  explanation  of  the  faculse.  Their 
spectrum  (as  long  ago  frequently  observed  by  the  writer, 
visually}  shows  the  great  H  and  K  bands  of  calcium 
always  reversed  by  a  thin  bright  line  running  down  the 
middle  of  each ;  and  while  the  reversal  directly  over  a 
spot  is  usually  u  single,"  it  is  usually  "  double  "  *  in  the 
faculous  region  surrounding  it — i.  e.,  the  bright  line  is 
itself  double,  as  in  Fig.  73,  page  231. 

This  makes  it  more  or  less  probable  that  the  faculse, 
instead  of  being  mere  protrusions  from  the  photosphere, 
are  really  luminous  masses  of  calcium  vapor  floating 
in  the  solar  atmosphere — possibly  identical  with  the 
prominences  themselves. 

Fie.  81*. 


SPECTBOHELIOGKAPH  PHOTOGBAPH  OP  SUN'S  DISK,  WITH  FAOTJLJB. 


But  Deslandres  and  Maunder  dissent  from  this,  and 
say  that  while  these  objects  shown  by  the  spectroscope 
are  clearly  connected  with  the  prominences,  they  are 
as  clearly  not  identical  with  them. 

*  Such  double  reversal  is  a  very  common  phenomenon  in  laboratory 
experiments  upon  metallic  spectra. 


HO  THE  SUN. 

Fig.  31*  is  from  one  of  Professor  Hale's  spectro- 
heliographic  photographs,  made  by  the  apparatus  de- 
scribed in  Chap.  VI,  page  233. 

The  faculae  are  found  to  some  extent  over  the  whole 
surface  of  the  sun,  though  only  sparingly  in  the  polar  re- 
gions, but  they  are  especially  abundant  in  the  immediate 
neighborhood  of  spots,  as  Fig.  30  well  shows.  There  are, 
however,  numerous  faculae  without  neighboring  spots. 

Except  near  the  spots,  the  f aculae  change  form  and 
place,  for  the  most  part,  rather  slowly,  persisting  some- 
times for  several  days  without  any  very  apparent  alter- 
ation. Still,  close  observation  and  micrometric  measure- 
ment will  always  detect  some  movement  or  deforma- 
tion, even  within  an  interval  of  only  an  hour  or  two ; 
and  near  the  spots  the  changes  are  often  so  rapid  and 
extreme  as  to  puzzle  even  a  skilled  draughtsman  to 
keep  up  with  them. 

This,  of  course,  shows  that  the  faculae  are  not  to  be 
identified  with  mountains  ;  they  are  not  permanent  and 
stable,  nor  is  the  surface  of  the  sun  continental  or  oce- 
anic even,  but  either  a  sheet  of  flame  or  of  cloud  rolling 
and  tossing,  and  never  at  rest.  When  we  come  to  study 
the  minute  details  of  the  granulation,  we  find  move- 
ments at  the  rate  of  a  thousand  miles  an  hour  to  be  the 
rule  rather  than  the  exception. 

And,  although  this  is  not  the  proper  place  to  treat 
the  subject  at  length,  we  may  add  that  all  we  can  learn 
as  to  the  temperature  and  constitution  of  the  sun  makes 
it  hardly  less  than  certain  that  the  visible  surface,  which 
is  called  the  photosphere,  is  just  a  sheet  of  self-luminous 
cloud  ;  precisely  like  the  clouds  of  our  own  atmosphere, 
with  the  exception  that  the  droplets  of  water  which 
constitute  terrestrial  clouds  are  replaced  on  the  sun  by 
drops  of  molten  metal,  and  that  the  solar  atmosphere  in 


7  h.  37  m. 


PHOTOGRAPHS    OF    A    PORTION    OF    THE    SUN, 

BY     JANSSEN. 
Meudon,  June  1,  1878,  Interval,  50  minutes. 


SUN-SPOTS  AND   THE  SOLAR  SURFACE.  HI 

which  thej  float  is  the  flame  of  a  burning,  fiery  furnace, 
raging  with  a  fury  and  an  intensity  beyond  all  human 
conception.  Looking  at  it  ninety-three  million  miles 
away,  we  fail  at  first  to  see,  in  such  objects  as  f  aculse  and 
granules,  the  evidence  of  such  commotion ;  but,  when 
we  convert  our  inicrometric  measurements  of  barely 
perceptible  changes  into  miles  and  velocities,  and  fig- 
ure to  ourselves  the  scale  of  movement,  we  gradually 
comprehend  their  meaning,  and  begin  to  understand 
what  we  are  dealing  with. 

A  great  advance  in  our  knowledge  of  the  structure 
of  the  solar  surface  was  gained  through  the  photograph- 
ic work  of  Janssen,  mentioned  in  a  previous  chapter.* 
Many  of  his  pictures  (in  which  the  disk  of  the  sun 
measures  about  eighteen  inches  in  diameter)  show  the 
details  of  the  surface  nearly  if  not  quite  as  well  as  any 
visual  observations ;  and  with  the  advantage  that,  while 
the  observer  with  the  eye  could  only  command  a  small 
field  of  view,  one  can,  on  these  photographic  plates, 
command  the  whole  at  once,  and  catch  the  relations  of 
different  parts.  On  examining  one  of  these  magnifi- 
cent plates,  one  is  at  first  struck  with  a  sort  of  "  smudg- 
iness"  (to  use  the  expression  of  Mr.  Huggins  in  de- 
scribing them),  which  might  give  the  impression  that 
it  was  not  properly  cleaned  before  coating  with  the 
collodion.  A  closer  examination,  however,  shows  that 
the  peculiarity  is  not  in  the  plate  but  in  the  image ; 
there  are  patches  of  clear  definition,  half  an  inch  or  so 
in  diameter  upon  a  picture  of  the  size  mentioned,  sep- 
arated by  streaks  and  patches  where  everything  is  in- 
distinct and  confused. 

One  might  naturally  attribute  this  to  the  disturbance 
of  the  air  in  the  telescope-tube,  and  to  clouds  of  vapor 

*  See  page  52. 


112  THE   SUN. 

rising  from  the  damp  collodion  surface  when  struck  by 
the  flash  of  sunlight  during  its  exposure ;  but  Janssen 
has  found  that  pictures  taken  in  immediate  succession 
show  the  same  "smudges"  on  the  same  parts  of  the 
sun,  which,  of  course,  would  not  happen  if  they  were 
the  result  of  accidental  currents  of  air  or  vapor  in  the 
telescope-tube.  He  infers,  therefore,  that  the  phenome- 
non is  solar,  and  has  given  it  the  name  of  the  Heseau 
Photospherique,  or  "  Photospheric  Reticulation,"  since 
the  streaks  and  patches  of  indistinctness  cover  the  sur- 
face like  a  net. 

The  discovery  of  this  feature  in  the  structure  of  the 
solar  surface  is  among  the  most  interesting  and  im- 
portant results  of  astronomical  photography. 

While  pictures  taken  in  immediate  succession  ex- 
hibit the  same  details  of  reticulation,  those  taken  at 
intervals  of  an  hour  or  two  show  great  changes,  es- 
pecially near  spots  and  faculae.  We  present  on  the 
opposite  page  a  pair  of  such  photographs,  borrowed 
from  the  "  Annuaire  "  of  the  Bureau  of  Longitudes  for 
1879.  The  original  pictures  were  taken  by  Janssen,  at 
Meudon,  on  June  1,  18T8,  with  an  interval  of  fifty  min- 
utes between  them.  They  show  clearly  the  peculiar 
characteristics  'of  the  reseau  photospherique,  as  well  as 
the  nature  and  extent  of  the  changes  which  occur  in  so 
short  a  time.  Compare,  especially,  the  granulation  in 
the  lower  right-hand  corner  of  each  picture,  and  imme- 
diately around  the  upper  spot,  remembering  all  the 
while  that  the  scale  of  the  picture  is  about  forty-six 
thousand  miles  to  the  inch,  and  that  the  little  spot  at 
the  top  of  the  figure  is  nearly  seven  thousand  miles  in 
diameter. 

The  idea  of  M.  Janssen  is  that  the  regions  of  indis- 
tinctness are  those  where  we  look  down  upon  the  sur- 


SUN-SPOTS  AND  THE  SOLAR  SURFACE.  H3 

face  through  a  portion  of  the  sun's  atmosphere  which 
is  at  the  moment  especially  agitated,  while  the  parts 
where  the  details  of  the  granulation  are  clear  and  well 
defined  are  those  which,  at  the  moment,  are  covered 
by  an  atmosphere  unusually  quiet  and  homogeneous. 
These  regions  are  continually  interchanging  with  each 
other,  just  as  areas  of  storm  and  fine  weather  sweep  over 
the  surface  of  the  earth,  but  with  inconceivably  greater 
swiftness. 

It  is  not,  however,  certain  that  the  disturbed  por- 
tions of  the  solar  atmosphere,  which  produce  the  in- 
distinctness in  question,  lie  near  the  sun's  surface.  It 
may  be  that  they  are  high  up,  and  it  would  not  be  an  un- 
reasonable conjecture  to  suppose  that  the  streamers  and 
luminous  masses  of  the  corona  may  be  concerned  in  the 
phenomenon ;  it  is  almost  certain  that  any  great  aggrega- 
tion of  chromospheric  matter  would  modify  the  appear- 
ance of  whatever  might  be  situated  beneath  it.  The 
simple  fact  is,  of  course,  that  we  are  looking  down  upon 
the  granules  and  other  features  of  the  sun's  surface,  not 
through  an  atmosphere  shallow,  cool,  and  quiet  like  the 
earth's,  but  through  an  envelope  of  matter,  partly  gase- 
ous and  partly,  perhaps,  pulverulent  or  smoke-like,  many 
thousand  miles  in  depth,  and  always  most  profoundly 
and  violently  agitated. 

But,  if  there  happens  to  be  a  well-formed  group  of 
spots  upon  the  solar  surface,  they  will  be  sure  to  claim 
the  attention  of  one  who,  for  the  first  time,  looks  at  the 
sun  through  the  telescope,  quite  to  the  exclusion  of 
everything  else.  The  umbra,  with  its  central  nuclei, 
and  overlying  bridges,  veils,  and  clouds ;  the  penumbra, 
with  its  delicate  structure  of  filaments  and  plumes ;  the 
surrounding  f aculas  and  the  agitated  surface  of  the  pho- 
tosphere in  the  whole  neighborhood  of  the  disturbance ; 


114  THE  SUN. 

above  all,  the  continual  change  and  progress  of  phe- 
nomena— combine  to  make  a  fine  sun-spot  one  of  the 
most  beautiful  and  intensely  interesting  of  telescopic 
objects. 

Even  before  the  days  of  telescopes  there  are  numer- 
ous records  of  dark  spots  seen  by  the  naked  eye  upon 
the  disk  of  the  sun,  especially  in  the  annals  of  the  Chi- 
nese. In  the  year  807  A.  D.,  a  large  spot  was  visible  in 
Europe  for  some  eight  days,  and  was  supposed  by  many 
to  be  the  planet  Mercury,  as  was  the  case  with  a  spot 
observed  by  Kepler  in  1609  ;  indeed,  in  all  cases  where 
such  appearances  were  noted,  they  were  invariably  as- 
cribed to  bodies  intervening  between  the  earth  and  the 
sun.  The  idea  of  such  imperfections  upon  the  disk  of 
a  celestial  body  was  utterly  repugnant  to  the  theologi- 
cal philosophy  of  the  middle  ages,  and  was  admitted 
only  slowly  and  grudgingly  even  after  the  demonstra- 
tion of  the  fact  was  complete. 

In  1610  and  1611  the  discovery  seems  to  have  been 
made  independently  by  Fabricius,  Scheiner,  and  Gali- 
leo— Fabricius,  according  to  our  modern  rules  of  scien- 
tific priority,  being  entitled  to  the  credit  as  the  first  to 
publish  the  fact  in  a  work,  "  De  Maculis  in  Sole  Obser- 
vatis,"  which  appeared  at  Wittenberg  in  June,  1611. 
The  discovery  was,  of  course,  a  necessary  corollary  to 
the  invention  of  the  telescope,  which  first  came  into  use 
in  Holland  in  1608  or  1609.  Fabricius's  first  observa- 
tion was  made  in  December,  1610.  Galileo,  in  a  letter 
responding  to  the  account  of  Scheiner's  discovery,  and 
published  early  in  1612,  claims  to  have  seen  the  sun- 
epots  with  his  newly-constructed  telescope  as  early  as 
October,  1610.  Scheiner  appears  to  have  first  seen  sun- 
spots  at  Ingolstadt  in  March,  1611 ;  but  his  ecclesiasti- 
cal superior  warned  him  against  believing  his  own  eyes 


SUN-SPOTS  AND   THE  SOLAR  SURFACE.  H5 

in  opposition  to  the  authority  of  Aristotle,  and  it  was 
not  until  November  and  December  that  he  published 
an  account  of  the  matter  in  three  letters  to  one  Welser, 
a  burgomaster  of  Augsburg,  some  months  after  the  work 
of  Fabricius  had  been  printed.  There  is  no  reason 
whatever  to  doubt  the  word  of  Galileo,  and  his  experi- 
ence in  losing  the  credit  of  this  discovery,  in  conse- 
quence of  his  slowness  of  publication,  seems  to  have 
been  the  origin  of  his  curious  method  of  publishing  his 
subsequent  discoveries  in  the  form  of  anagrams,  the  in- 
terpretation of  which  was  withheld  for  a  time. 

At  the  very  outset  of  his  observations,  Fabricius,  as 
well  as  Galileo,  recognized  that  the  spots  are  objects 
upon  the  surface  of  the  sun,  and  that  this  body  rotates 
on  its  axis,  carrying  them  with  it.  Scheiner  at  first 
maintained  that  they  were  planets  moving  very  near 
the  sun,  but  not  in  contact  with  it.  Many  shared  this 
opinion,  and  Tarde,  a  French  astronomer,  went  so  far 
as  to  name  them  the  Bourbonian  stars,  in  honor  of  the 
Bourbon  dynasty.  Scheiner's  further  observations  soon 
convinced  him,  however,  of  the  correctness  of  Galileo's 
opinion  and  arguments.  Some  twenty  years  later 
Scheiner  published  an  enormous  volume,  the  "Rosa 
IJrsina,"  containing  an  account  of  his  observations  and 
apparatus.  His  telescope  was  mounted  equatorially,  and 
arranged  to  throw  the  sun's  image  upon  a  screen  in  pre- 
cisely the  manner  employed  by  some  of  the  best  mod- 
ern observers.  He  determined  the  time  of  the  sun's 
rotation  and  the  position  of  his  equator  with  a  very 
creditable  degree  of  accuracy. 

Since  then  observations  upon  these  objects  have 
been  more  or  less  kept  up  all  the  time,  but  not  with 
any  regular  assiduity  until  within  the  last  thirty  years. 
It  was  soon  found  that  they  are  only  transitory  and 


116  THE  SUN. 

cloud-like  in  their  nature,  and  interest  in  them  there 
fore  flagged,  until  their  relations  to  the  constitution  oi 

C?O          * 

the  sun  began  to  be  recognized. 

A  well-formed  solar  spot  consists,  generally  speak* 
ing,  of  two  portions— a  very  dark,  irregular,  central 
portion  called  the  umbra,  surrounded  by  a  shade  01 
fringe  called  the  penumbra,  less  dark,  and  for  the  most 
part  made  up  of  filaments  directed  radially  inward. 
The  appearance  of  things,  under  ordinary  circumstances 
of  seeing,  is  as  if  the  umbra  were  a  hole,  and  the  pe- 
numbral  filaments  overhung  and  partly  shaded  it  from 
our  view,  like  bushes  at  the  mouth  of  a  cavern.  I  say 
as  if,  and  very  possibly  this  is  the  actual  case,  the  cen- 


FlG. 


SPOT  OF  JULY  16,  1866. 


tral  portion  being  a  real  cavity  filled  with  less  luminous 
matter,  and  depressed  below  the  general  level  of  the 
photosphere,  while  the  penumbra  overhangs  the  edge. 

The  figure,  copied  from  Secchi,  is  a  fair  represen- 
tation of  such  a  spot,  and  may  be  compared  with  the 


SUN-SPOTS  AND  THE  SOLAR  SURFACE. 

photographs  of  Janssen,  which  exhibit  pretty  much  the 
same  peculiarities,  though  with  less  of  minute  detail. 
The  drawings  of  Nasmyth  and  Langley  *  show  so  much 
more  of  the  detail  than  is  ordinarily  seen,  that  they  are 
really  less  satisfactory  representations  of  what  one  may 
expect  when  he  observes  a  spot  for  the  first  time.  Sev- 
eral points  at  once  strike  the  attention.  In  the  first 
place,  the  nearly  circular  form  of  the  spot,  which  is  the 
ordinary  form  during  the  middle  life  of  one  of  these 
objects.  While  forming,  and  when  on  the  point  of  dis- 
appearing, it  is  usually  much  more  irregular.  It  is  to 
be  noticed  also  that  there  is  nothing  like  a  gradual 
shading  off,  either  between  the  umbra  and  the  penum- 
bra or  between  the  penumbra  and  the  surrounding  por- 
tions of  the  photosphere ;  on  the  contrary,  the  line  of 
separation  is  strongly  marked  in  each  case,  the  penum- 
bra being  much  brighter  at  the  inner  edge,  and  darker 
at  the  outer,  so  that  it  contrasts  distinctly  both  with  the 
umbra  and  with  the  neighboring  surface  of  the  sun. 
This  brightness  of  the  inner  penumbra  seems  to  be  due 
to  the  crowding  together  of  the  penumbral  filaments 
where  they  overhang  the  umbra.  Again,  it  is  observ- 
able that  there  is  a  general  antithesis  between  the  irreg- 
ularities of  the  contour  of  the  outer  and  inner  edges  of 
the  penumbra.  For  the  most  part,  where  an  angle  of 
the  penumbral  matter  crowds  in  upon  the  umbra,  it  is 
matched  by  a  corresponding  outward  extension  into  the 
photosphere,  and  vice  versa.  It  is  noticeable  also  that 
many  of  the  penumbral  filaments  are  terminated  by 
little  detached  grains  of  luminous  matter,  and  there  are 
also  fainter  veils  of  a  substance  less  brilliant,  but  some- 
times rose-colored,  which  seem  to  float  above  the  um- 
bra. Otherwise  the  umbra  in  the  figure  appears  to  be 

*  See  frontispiece,  and  page  104. 


THE 

uniformly  dark  ;  *  but,  if  we  had  been  actually  observing 
the  object  on  the  16th  of  July,  1866,  when  this  pict- 
ure was  made,  we  should  have  found  even  the  umbra 
full  of  detail — made  up  of  cloudy  masses  of  a  brilliance 
really  intense,  and  dark  only  by  contrast  with  the  still 
intenser  brightness  of  the  solar  surface,  as  becomes  ap- 
parent when  the  light  from  other  portions  is  excluded. 
Probably  we  should  have  been  able  also  to  detect 
among  these  clouds  one  or  more  of  the  minute  circular 
spots,  first  discovered  by  Dawes,  much  darker  than  the 
rest  of  the  umbra,  and  looking  like  the  mouths  of  tubu- 
lar orifices  penetrating  to  unknown  depths. 

If  we  were  able  to  continue  our  watch  for  some 
time,  we  should  see  the  details  continually  changing. 
The  faint  veils  of  overlying  cirrus  would  probably  melt 
away,  and  be  replaced  by  others  in  some  different  po- 
sition ;  the  bright  granules  at  the  tips  of  the  penumbral 
filaments  would  seem  to  sink  and  dissolve,  and  fresh 
portions  would  break  off  to  replace  them.  We  should 
find  a  continual  indraught  of  the  luminous  matter  over 
the  whole  extent  of  the  penumbra.  Almost  certainly 
the  spot  would  change  its  form  and  size,  quite  percep- 
tibly from  day  to  day,  and  sometimes  even  from  hour 
to  hour.  Of  course,  we  should  find  it  steadily  moving 
over  the  solar  disk  from  the  east  toward  the  west,  and 
as  it  neared  the  edge  it  would  become  apparently  ellip- 

*  The  umbra  appears  not  black,  but  of  a  deep  purplish  tint.  It  is 
questionable,  however,  whether  this  color  is  real,  or  only  due  to  the  sec- 
ondary spectrum  of  the  telescope  object-glass.  The  principal  reason  for 
suspecting  this  to  be  the  case  is  in  the  fact  that,  during  a  transit  of 
Mercury  or  Venus,  the  planet's  disk  is  found  to  present  precisely  the 
same  tint,  while  there  is  no  imaginable  explanation  for  its  really  being 
anything  but  black.  It  is  certain,  too,  on  optical  grounds,  that  any 
ordinary  object-glass  must  show  a  purplish  fringe  extending  inward  over 
any  dark  spot  upon  a  white  background. 


SUN-SPOTS  AND   THE   SOLAR  SURFACE.  H9 

tical  in  form ;  the  penumbra  on  the  edge  of  the  spot 
nearest  the  center  of  the  sun  would  grow  narrower 
and,  perhaps,  disappear  entirely,  and  at  last  the  spot, 
appearing  like  a  mere  line  of  darkness,  but  probably 
accompanied  by  an  attendant  crowd  of  faculse,  would 
pass  out  of  sight  behind  the  limb,  perhaps  to  reappear 
again  after  a  fortnight  at  the  eastern  edge.  I  say  per- 
haps, because,  quite  as  often  as  not,  these  short-lived 
objects  are  seen  but  once,  not  lasting  through  even  a 
single  revolution  of  the  sun. 

The  average  life  of  a  sun-spot  may  be  taken  as  two 
or  three  months ;  the  longest  yet  on  record  is  that  of 
a  spot  observed  in  1840  and  1841,  which  lasted  eigh- 
teen months.  There  are  cases,  however,  where  the  dis- 
appearance of  a  spot  is  very  soon  followed  by  the  ap- 
pearance of  another  at  the  same  point,  and  sometimes 
this  alternate  disappearance  and  reappearance  is  several 
times  repeated.  While  some  spots  are  thus  long-lived, 
others,  however,  endure  only  for  a  day  or  two,  and 
sometimes  only  for  a  few  hours. 

The  spots  usually  appear  not  singly,  but  in  groups — 
at  least,  isolated  spots  of  any  size  are  less  common  than 
groups.  Very  often  a  large  spot  is  followed  upon  the 
eastern  side  by  a  train  of  smaller  ones ;  many  of  which, 
in  such  a  case,  are  apt  to  be  very  imperfect  in  structure, 
sometimes  showing  no  umbra  at  all,  often  having  a  pe- 
numbra only  upon  one  side,  and  usually  irregular  in 
form.  It  is  noticeable,  also,  that  in  such  cases,  when 
any  considerable  change  of  form  or  structure  shows 
itself  in  the  principal  spot  of  a  group,  it  seems  to  rush 
forward  (westward)  upon  the  solar  surface,  leaving  its 
attendants  trailing  behind.  When  a  large  spot  divides 
into  two  or  more,  as  often  happens,  the  parts  usually 
seem  to  repel  each  other  and  fly  asunder  with  great 


120  THE  SUN. 

velocity — great,  that  is,  if  reckoned  in  miles  per  hour, 
though,  of  course,  to  a  telescopic  observer  the  motion 
is  very  slow,  since  one  can  only  barely  see  upon  the 
Bun's  surface  a  change  of  place  amounting  to  two  hun- 
dred miles,  even  with  a  very  high  magnifying  power. 
Velocities  of  three  or  four  hundred  miles  an  hour  are 
usual,  and  velocities  of  one  thousand  miles,  and  even 
more,  are  by  no  means  exceptional. 

At  times,  though  very  rarely,  a  different  phenome- 
non of  the  most  surprising  and  startling  character  ap- 
pears in  connection  with  these  objects :  patches  of  in- 
tense brightness  suddenly  break  out,  remaining  visible 
for  a  few  minutes,  moving,  while  they  last,  with  veloci- 
ties as  great  as  one  hundred  miles  a  second. 

One  of  these  events  has  become  classical.  It  oc- 
curred on  the  forenoon  (Greenwich  time)  of  Septem- 
ber 1,  1859,  and  was  independently  witnessed  by  two 
well-known  and  reliable  observers,  Mr.  Carrington  and 
Mr.  Hodgson,  whose  accounts  of  the  matter  may  be 
found  in  the  monthly  notices  of  the  Royal  Astronomi- 
cal Society  for  November,  1859.  Mr.  Carrington  at 
the  time  was  making  his  usual  daily  observation  upon 
the  position,  configuration,  and  size  of  the  spots  by 
means  of  an  image  of  the  solar  disk  upon  a  screen, 
being  then  engaged  upon  that  eight  years'  series  of 
observations  which  lies  at  the  foundation  of  so  much 
of  our  present  solar  science.  Mr.  Hodgson,  at  a  dis- 
tance of  many  miles,  was  at  the  same  time  sketching 
details  of  sun-spot  structure  by  means  of  a  solar  eye- 
piece and  shade-glass.  They  simultaneously  saw  two 
luminous  objects,  shaped  something  like  two  new  moons, 
each  about  eight  thousand  miles  in  length  and  two  thou- 
sand wide,  at  a  distance  of  some  twelve  thousand  miles 
from  each  other.  These  burst  suddenly  into  sight  at 


SUN-SPOTS  AND   THE  SOLAR  SURFACE.  121 

the  edge  of  a  great  sun-spot,  with  a  dazzling  brightness 
at  least  five  or  six  times  that  of  the  neighboring  por- 
tions of  the  photosphere,  and  moved  eastward  over  the 
spot  in  parallel  lines,  growing  smaller  and  fainter,  until 
in  about  five  minutes  they  disappeared,  after  traversing 
a  course  of  nearly  thirty-six  thousand  miles.  Their  pas- 
sage did  not  seem  in  any  way  to  change  the  configura- 
tion of  the  spot  over  which  they  passed.  Mr.  Carring- 
ton  found  his  drawing,  which  was  completed  just  before 
they  appeared,  still  quite  correct  after  they  had  vanished. 
Of  course,  it  is  possible  to  question  the  connection  be- 
tween this  phenomenon  and  the  spot  near  which  it  ap- 
peared ;  but,  as  somewhat  similar  appearances  have  been 
seen  by  other  observers  since  then,  and  always  in  the 
neighborhood  of  spots,  it  is  probable  that  there  is  some 
relation  in  the  case.  Opinions  have  differed  widely  as 
to  the  explanation.  Some  have  maintained  that  the 
phenomenon  was  simply  due  to  the  fall  of  a  couple  of 
immense  meteors  into  the  sun's  atmosphere,  others  that 
it  was  caused  by  some  sudden  and  powerful  eruption 
from  beneath,  such  as  the  spectroscope  often  reveals  to 
us  nowadays;  an  eruption,  however,  of  most  unusual 
brilliance  and  violence,  for  not  one  of  the  outbursts  since 
then  observed  by  the  spectroscope  has  ever  been  visible 
without  its  aid. 

The  event  occurred  in  the  midst  of  a  remarkable 
magnetic  storm :  from  August  28th  to  September  4th 
there  were  auroras  every  night  all  over  the  world,  and 
the  earth  currents  were  often  so  strong  as  greatly 
to  interfere  with  telegraphic  communication.  On  the 
night  of  September  1st,  however,  as  Mr.  Ellis  has  lately 
shown  from  the  original  records,  the  magnetic  disturb- 
ance was  not  specially  intense,  so  that  the  occurrence 
observed  by  Carrington  and  Hodgson  could  not  have 


122  THE 

been  the  cause  of  the  magnetic  storm — more  likely  it 
was  a  consequence,  if  there  was  any  connection. 

There  is  no  regular  process  for  the  formation  of  a 
spot.  Sometimes  it  is  gradual,  requiring  days  or  even 
weeks  for  its  full  development,  and  sometimes  a  single 
day  suffices.  Generally,  for  some  time  before  the  ap- 
pearance of  the  spot,  there  is  an  evident  disturbance  of 
the  solar  surface,  manifested  especially  by  the  presence 
of  numerous  and  brilliant  faculse,*  among  which, 
"pores"  or  minute  black  dots  are  scattered.  These 
enlarge,  and  between  them  appear  grayish  patches,  ap- 
parently caused  by  a  dark  mass  lying  veiled  below  a 
thin  layer  of  luminous  filaments.  The  veil  grows  grad- 
ually thinner,  and  vanishes,  giving  us  at  last  the  com- 
pleted spot  with  its  perfect  penumbra.  The  "  pores," 
some  of  them,  coalesce  with  the  principal  spot,  some 
disappear,  and  others  constitute  the  attendant  train. 
When  the  spot  is  once  completely  formed,  it  assumes 
usually  an  approximately  circular  form,  and  remains 
without  striking  change  until  its  dissolution.  As  its 
end  approaches,  the  surrounding  photosphere  seems  to 
crowd  in  upon  and  cover  and  overwhelm  the  penumbra. 
Bridges  of  light,  often  many  times  brighter  than  the 
average  of  the  solar  surface,  push  across  the  umbra,  the 
arrangement  of  the  penumbra  filaments  becomes  con- 
fused, and,  as  Secchi  expresses  it,  the  luminous  matter 
of  the  photosphere  seems  to  tumble  pell-mell  into  the 
chasrn,  which  disappears  and  leaves  a  disturbed  surface 
marked  with  faculse,  which  in  their  turn  subside  after  a 
time.  As  intimated  before,  however,  the  disturbance  is 
not  unfrequently  renewed  at  the  same  point  after  a  few 
days,  and  a  fresh  spot  appears  just  where  the  old  one 
was  overwhelmed. 

*  This  is  Secchi's  view,     Lockyer  maintains  that  the  spots  appear  be- 
fore the  f  aculae. 


SUN-SPOTS  AXD  THE  SOLAR  SURFACE.  123 

We  transcribe  from  a  paper  by  the  late  Dr.  Peters, 
of  Hamilton  College,  a  very  graphic  account  of  the  ap- 
pearance and  decay  of  certain  sun-spots,  based  upon  his 
observations  at  Naples  in  1845-'46.  It  is  printed  in 
Volume  IX  of  the  "  Proceedings  of  the  American  As- 
sociation for  the  Advancement  of  Science."  He  says : 

"  The  spots  arise  from  insensible  points,  so  that  the  exact 
moment  of  their  origin  can  not  be  stated;  but  they  grow  very 
rapidly  in  the  beginning,  and  almost  always  in  less  than  a  day 
they  arrive  at  their  maximum  of  size.  Then  they  are  stationary, 
I  would  say  in  the  vigorous  epoch  of  their  life,  with  a  well-defined 
penumbra  of  regular  and  rather  simple  shape.  So  they  sustain 
themselves  for  ten,  twenty,  and  some  even  for  fifty  days.  Then 
the  notches  in  the  margin,  which,  with  a  high  magnifying  power, 
always  appear  somewhat  serrate,  grow  deeper,  to  such  a  degree 
that  the  penumbra  in  some  parts  becomes  interrupted  by  straight 
and  narrow  luminous  tracks — already  the  period  of  decadence  is 
approaching.  This  begins  with  the  following  highly  interesting 
phenomenon:  Two  of  the  notches  from  opposite  sides  step  for- 
ward into  the  area,  over-roofing  even  a  part  of  the  nucleus ;  and 
suddenly  from  their  prominent  points  flashes  go  out,  meeting 
each  other  on  their  way,  hanging  together  for  a  moment,  then 
breaking  off  and  receding  to  their  points  of  starting.  Soon  this 
electric  play  begins  anew  and  continues  for  a  few  minutes,  ending 
finally  with  the  connection  of  the  two  notches,  thus  establishing  a 
bridge,  and  dividing  the  spot  in  two  parts.  Only  once  I  had  the 
fortune  to  witness  the  occurrence  between  three  advanced  points. 
Here,  from  the  point  A  a  flash  proceeded  toward  B,  which  sent 
forth  a  ray  to  meet  the  former  when  this  had  arrived  very  near. 
Soon  this  seemed  saturated,  and  was  suddenly  repelled  ;  however, 
it  did  not  retire,  but  bent  with  a  sudden  swing  toward  0 ;  then 
again,  in  the  same  manner,  as  by  repulsion  and  attraction,  it  re- 
turned to  B  ;  and,  after  having  thus  oscillated  for  several  times,  A 
adhered  at  last  permanently  to  B.  The  flashes  proceeded  with 
great  speed,  but  not  so  that  the  eye  might  not  follow  them  dis- 
tinctly. By  an  estimation  of  time  and  the  known  dimension  of 
space  traversed,  at  least  an  under  limit  of  the  velocity  may  be 
found ;  thus,  I  compute  this  velocity  to  be  not  less  than  two  hun- 


124  THE  SUN. 

dred  millions  of  metres  (or  about  one  hundred  and  twenty  thou- 
sand miles)  in  a  second  (sic). 

"  The  process  described  is  accomplished  in  the  higher  photo- 
sphere, and  seems  not  to  affect  at  all  the  lower  or  dark  atmos- 
phere. With  it  a  second,  or  rather  a  third,  period  in  the  spot's 
life  has  begun,  that  of  dissolution,  which  lasts  sometimes  for  ten 
or  twenty  days,  during  which  time  the  components  are  again  sub- 
divided, while  the  other  parts  of  the  luminous  margin,  too,  are 
pressing,  diminishing,  and  finally  overcasting  the  whole,  thus  end- 
ing the  ephemeral  existence  of  the  spot. 

"Rather  a  good  chance  is  required  for  observing  the  remark- 
able phenomenon  which  introduces  the  covering  process,  since  it 
is  achieved  in  a  few  minutes,  and  it  demands,  moreover,  a  per- 
fectly calm  atmosphere,  in  order  not  to  be  confounded  with  a 
kind  of  scintillation  which  is  perceived  very  often  in  the  spots, 
especially  with  fatigued  eyes.  The  observer  ought  to  watch  for 
it  under  otherwise  favorable  circumstances  when  a  large  and  ten- 
or twenty-days'-old  spot  begins  to  show  strong  indentations  on 
the  margin." 

Dr.  Peters,  so  far  as  we  know,  is  the  only  observer 
who  describes  the  remarkable  phenomenon  of  flashes 
extending  across  an  umbra  with  electrical  velocity  ;  and 
for  this  reason,  and  because  his  instrument  was  not  of 
the  highest  power — a  three-and-a-half -inch  refractor — 
perhaps  his  account  must  be  received  with  a  little  re- 
serve until  further  confirmed.  At  the  same  time,  there 
is  nothing  in  the  nature  of  the  sun,  or  of  a  sun-spot,  so 
far  as  at  present  known,  to  make  the  statement  in  itself 
impossible ;  and  certainly  Dr.  Peters  holds  deservedly 
a  very  high  rank  among  astronomers  for  acuteness  and 
accuracy  of  observation  and  description. 

It  must  not  be  understood  that  the  life  history  of  a 
spot,  just  sketched,  applies  to  all,  or  even  with  exact- 
ness to  a  majority,  of  them.  Almost  every  one  has  its 
own  idiosyncrasies,  departing  in  some  respect  or  other 
from  the  usual  course  of  things.  Spots  of  unusual  mag- 


SUN-SPOTS  AND   THE  SOLAR  SURFACE.  125 

nitude  and  activity  often  seem  to  have  no  quiet  middle 
life ;  there  is  no  time  in  their  history  when  they  are  not 
doing  something  or  other  surprising,  and  more  or  less 
unprecedented. 

We  have  spoken  of  the  filaments  which  compose 
the  penumbra  as  directed  inward  toward  the  center  of 
the  spot.  This  is  the  general  rule,  but  the  exceptions 
are  numerous,  and  nothing  can  show  better  than  Pro- 
fessor Langley's  exquisite  drawing  how  wide  the  di- 
vergence often  is  from  this  law.  While  at  the  left- 
hand  and  upper  portions  of  the  great  spot  (which, 
though  "  typical,"  is  not  a  specimen  of  a  quiescent  spot) 
the  filaments  present  the  ordinary  appearance,  at  the 
lower  edge  and  upon  the  great  overhanging  branch 
they  are  arranged  very  differently.  Very  curious,  and 
rare,  also,  though  we  have  ourselves  seen  a  similar  thing 
on  two  or  three  occasions,  is  the  feathery  brush  which 
reaches  in  below  the  "  branch,"  so  closely  resembling  a 
frost-crystal  upon  the  window-pane  in  a  winter's  morn- 
ing. What  may  be  the  cause  of  such  formations  it  is 
now  quite  impossible  to  say.  Probably  analogies  drawn 
from  our  terrestrial  clouds  will  go  further  toward  an 
explanation  than  any  others  yet  proposed. 

Usually  the  penumbral  filaments  are  brightest  at  the 
inner  end  where  they  apparently  project  over  the  um- 
bra, and  under  ordinary  circumstances  of  vision  the  end 
appears  blunt  and  even  club-shaped. 

With  the  great  twenty-three-inch  telescope  at  Prince- 
ton, and  on  a  few  occasions,  when  the  seeing  has  been 
fine  enough  to  permit  the  use  of  powers  of  from  six  hun- 
dred and  upward,  the  writer  has  found  that,  in  many 
cases  at  least,  the  apparently  club-like,  almost  bulbous, 
ends  of  the  penumbral  filaments  are  really  fine,  sharp- 
pointed  hooks,  reminding  one  of  the  curling  tips  of 


126  THE 

flames,  or  grass-blades  bending  over.  Ordinarily  they 
are  seen  as  club-like,  simply  because  of  their  brightness 
and  the  irradiation  and  diffraction  effects  of  moderate- 
sized  object-glasses. 

Not  unf requently  the  penumbral  filaments  are  curved 
and  spirally  arranged,  showing  a  marked  cyclonic  action. 
In  such  cases  the  whole  spot  usually  turns  slowly  around, 
sometimes  completing  an  entire  revolution  in  a  few  days. 
More  frequently,  however,  the  spiral  motion  persists 
but  a  short  time,  and  occasionally,  after  continuing  for 
a  while  in  one  direction,  the  motion  is  reversed.  Very 
often,  in  spots  of  considerable  extent,  there  are  opposite 
spiral  movements  in  different  portions  of  the  umbra ; 
indeed,  this  is  rather  the  rule  than  the  exception.  Neigh- 
boring spots  show  no  tendency  to  rotate  in  the  same 
direction.  The  number  of  spots  in  which  a  decided 
cyclonic  motion  appears  is  relatively  quite  small,  not 
exceeding,  according  to  the  observations  of  Carrington 
and  Secchi,  more  than  two  or  three  per  cent,  of  the 
whole.  Of  course,  these  facts  are  sufficient  to  show 
that  this  kind  of  motion,  when  it  occurs,  is  not  attribut- 
able to  anything  like  that  action  of  the  terrestrial  atmos- 
phere which  determines  the  right-  and  left-handed 
rotation  of  our  great  storms  in  the  southern  and  northern 
hemispheres.  It  is  probably  caused  in  sun-spots  by 
merely  accidental  circumstances  which  convert  the  pe- 
numbral indraught  into  a  whirl  of  no  great  rapidity  or 
certain  direction.  It  does  not  seem  possible  to  find  in 
this  occasional  cyclonic  motion,  as  Faye  attempts  to  do, 
the  key  and  explanation  of  the  whole  series  of  sun-spot 
phenomena. 

The  dimensions  of  sun-spots  are  sometimes  enor- 
mous. Many  groups  have  been  observed  covering 
areas  of  more  than  one  hundred  thousand  iniles  square. 


SUN-SPOTS  AND   THE  SOLAR  SURFACE.  127 

and  single  spots  have  been  known  to  measure  forty  or 
fifty  thousand  miles  in  diameter,  the  central  umbra 
alone  being  twenty-five  or  thirty  thousand  miles  across. 
A  spot,  however,  measuring  thirty  thousand  miles  over 
all,  would  be  considered  large  rather  than  small. 

An  object  of  this  size  upon  the  sun's  surface  can 
easily  be  seen  without  a  telescope  when  the  brightness 
is  reduced  either  by  clouds,  or  nearness  to  the  horizon, 
or  by  the  use  of  a  shade-glass.  At  the  transit  of  Venus, 
in  1882,  every  one  saw  the  planet  readily  without  tele- 
scopic aid.  Her  apparent  diameter  was  about  67"  at 
the  time,  which  is  equivalent  to  about  thirty  one  thou- 
sand miles  on  the  solar  surface.  Probably  a  very  keen 
eye  would  detect  a  spot  measuring  not  more  than  twenty- 
three  or  twenty-four  thousand  miles. 

Hardly  a  year  passes,  at  times  when  spots  are  numer- 
ous, without  furnishing  several  as  large  as  this ;  so  that 
it  is  rather  surprising  than  otherwise  that  we  have  not 
a  greater  number  of  sun-spot  records  in  the  pre  tele- 
scopic centuries.  The  explanation  probably  lies  in  two 
things :  the  sun  is  too  bright  to  be  often  or  easily  looked 
at,  and  when  spots  were  seen  they  would  be  likely  to 
be  taken  for  optical  illusions  rather  than  realities. 

During  the  years  1871  and  1872  spots  were  visible 
to  the  naked  eye  for  a  considerable  portion  of  the  time. 
On  several  occasions  pupils  of  the  writer  have  noticed 
them  of  their  own  accord,  without  having  had  their  at- 
tention previously  directed  to  the  matter. 

The  largest  spot  yet  recorded  was  observed  in  1858. 
It  had  a  breadth  of  more  than  one  hundred  and  forty- 
three  thousand  miles,  or  nearly  eighteen  times  the  diam- 
eter of  the  earth,  and  covered  about  one  thirty-sixth  of 
tile  whole  surface  of  the  sun.  Other  very  large  ones 
appeared  in  1892  and  1893. 


128 


THE  SUN. 


Fig.  33,  taken  by  the  publisher's  permission  from 
Flammarion's  Popular  Astronomy,  represents  a  very 


FIG.  33. 


ONE  OP  THE  LARGEST  STTN-SPOTS  :  SEVEN  TIMES  THE  SIZE  OF  THE  EARTH. 
OBSERVED  OCTOBER  14,  1883.    VISIBLE  TO  THE  NAKED  EYE. 

large  and  interesting  spot  which  appeared  in  October, 
1883.  It  is  from  the  drawings  of  Tacchini.  Spet- 
tros.  ItaL,  Vol.  XIII. 

NATURE    OF   THE    SPOTS. 

It  has  been  intimated  that  the  spots  are  depressions 
below  the  general  level  of  the  solar  surface.  For  more 
than  a  century  this  has  been  the  accepted  doctrine,  and 
it  is  probably  correct ;  at  the  same  time  it  can  hardly  be 


SUN-SPOTS   AND   THE   SOLAR  SURFACE.  129 

regarded  as  absolutely  settled,  since  it  has  been  called  in 
question  recently  by  high  authorities,  and  is  still  de- 
bated. In  December,  1894,  Mr.  Hewlett,  who  has  been 
for  more  than  thirty  years  a  persistent  observer  of  the 
sun,  presented  to  the  Royal  Astronomical  Society  all 
his  sun-spot  drawings,  several  thousand  in  number,  and 
covering  the  whole  period  from  1859  to  1893.  He  took 
the  opportunity  to  express  very  strongly  his  opinion 
that  the  facts  are  against  the  theory  that  the  spots  are 
" hollows"  as  usually  supposed,  and  was  supported  in 
his  view  by  a  number  of  good  observers,  who  made  it 
clear  that  if  the  spots  are  really  depressed  at  all,  they 
must  be  very  shallow  compared  with  their  diameter. 

The  idea  was  first  clearly  brought  out  by  Dr.  Wil- 
son, of  Glasgow,  in  1769,  and  his  demonstration  was 
based  upon  the  behavior  of  the  penumbra  of  a  spot 
which  he  observed  in  November  of  that  year.  He 
found  that,  when  the  spot  appeared  at  the  eastern  limb 
or  edge  of  the  sun,  just  moving  into  sight,  the  penum- 
bra was  well  marked  on  the  side  of  the  spot  nearest  to 
the  edge  of  the  disk,  while  on  the  other  edge  of  the 
spot,  that  next  the  center,  there  was  no  penumbra  vis- 
ible at  all,  and  the  umbra  itself  was  almost  hidden,  as 
if  behind  a  bank.  When  the  spot  had  moved  a  day's 
journey  farther  inward  toward  the  center  of  the  disk, 
the  whole  of  the  umbra  came  into  sight,  and  the  pe- 
numbra on  the  inner  edge  of  the  spot  began  to  be  visible 
as  a  narrow  line.  After  the  spot  was  well  advanced 
upon  the  disk,  the  penumbra  was  of  the  same  width  all 
around  the  spot ;  but,  when  the  spot  approached  the 
sun's  western  limb,  the  same  phenomena  were  repeated 
as  at  the  eastern — that  is,  the  penumbra  on  the  wine? 
edge  of  the  spot  narrowed  much  faster  than  that  on 
the  outer,  disappeared  entirely,  and  finally  seemed  to 
10 


130  THE  SUN. 

hide  from  sight  much  of  the  umbra,  nearly  a  whole  day 
before  the  spot  passed  from  view  around  the  limb.  Of 
course,  it  is  hardly  necessary  to  point  out  what  the  fig- 
ure at  once  makes  evident,  that  this  is  precisely  the  way 
things  would  go  if  the  spot  were  a  saucer-shaped  de 

FIG.  34. 


DIAGRAM  ILLUSTRATING  THE  FACT  THAT  SUN-SPOTS  ARE  HOLLOWS  IN  THK 
PHOTOSPHERE. 

pression  in  the  sun's  surface,  the  bottom  of  the  saucer 
corresponding  to  the  umbra,  and  the  sloping  sides  to 
the  penumbra. 

The  observation  of  a  single  spot  would  hardly 
settle  the  question,  because  we  frequently  have  spots 
witli  a  one-sided  penumbra.  In  fact,  when  spots  are 
either  in  the  process  of  formation  or  of  dissolution 
the  penumbra  is  seldom  of  uniform  width  all  around, 
De  La  Rue,  Stewart,  and  Loewy  made,  therefore,  some 
years  ago,  a  careful  discussion  of  something  more  than 
six  hundred  cases  of  spots,  with  measurable  penum- 
brse,  and  found  that,  in  a  little  over  seventy-five  per 
cent,  of  all  the  cases,  the  penumbra  was  widest  on  the 


SUN-SPOTS  AND   THE   SOLAR   SURFACE.  131 

edge  of  the  spot  nearest  the  limb,  as  Wilson's  theory 
requires ;  in  a  little  more  than  twelve  per  cent,  there 
was  no  noticeable  difference;  and  in  the  remaining 
twelve  per  cent,  it  was  widest  on  the  inner  edge. 
Father  Sidgreaves,  on  the  other  hand,  in  following  up 
the  discussion  raised  by  Mr.  Howlett,  gets  an  opposing 
verdict  from  the  Stonyhurst  sun-spot  drawings.  Out  of 
one  hundred  and  eighty-seven  sketches,  which  he  se- 
lected as  fair  tests  of  the  Wilsonian  theory,  only  forty- 
seven  favored  it,  and  one  hundred  and  forty  were 
opposed.  But  we  suppose  he  has  included  as  "  opposed  " 
all  that  did  not  distinctly  indicate  depression. 

Others,  Secchi  especially,  have  investigated  the  mat- 
ter by  carefully  measuring,  from  day  to  day,  the  position 
on  the  sun's  disk  of  some  selected  point  in  the  umbra  of 
a  spot.  The  work  is  not  easy,  and  rather  unsatisfactory, 
on  account  of  the  rapid  changes,  which  make  it  difficult 
to  identify  the  point  of  reference  in  successive  observa- 
tions ;  still,  the  result  appears  decisive,  showing,  as  an 
ordinary  rule,  that  what  may  be  called  the  "  floor  "  of 
the  umbra  is  depressed  from  two  to  six  thousand  miles, 
and  sometimes  more,  below  the  general  level  of  the 
photosphere.  But  the  refraction  of  the  solar  atmos- 
phere makes  the  result  uncertain. 

On  a  few  occasions,  when  a  spot  of  unusual  size  and 
depth  passes  over  the  limb  of  the  sun,  a  distinct  depres- 
sion is  observed  in  the  outline.  Cassini  describes  such 
an  instance  in  1719.  Herschel,  De  La  Rue,  Secchi,  and 
others  have  given  us  several  other  observations  of  the 
same  kind.  Usually,  however,  the  faculse,  which  sur- 
round the  spot,  mask  this  effect  entirely,  and  often 
actually  give  us  a  number  of  little  projecting  hillocks 
in  place  of  the  expected  depression. 


132  THE  SUN- 

SPECTRUM   OF    SUN-SPOTS. 

The  spectrum  of  a  sun-spot  also  furnishes  an  argu- 
ment in  the  same  direction,  tending  to  show  that  the 
dark  portion  is  a  cavity  filled  with  gases  and  vapors, 
which  produce  the  obscuration,  in  part,  at  least,  by  ab- 
sorbing the  light  emitted  from  the  floor  of  the  depres- 
sion. It  is  not  difficult  to  set  the  instrument  in  such  a 
manner  that  the  image  of  a  sun-spot  shall  fall  precisely 
upon  the  slit  of  the  spectroscope.  In  this  case  the  spec- 
trum will  be  seen  to  be  traversed  by  a  longitudinal  dark 
stripe,  which  is  the  spectrum  of  the  umbra  of  the  spot : 
on  each  side  is  the  spectrum  of  the  penumbra,  which  is 
usually  only  a  trifle  fainter  than  that  of  the  general  sur- 
face of  the  sun.  The  width  of  the  stripe,  of  course,  de- 
pends upon  the  diameter  of  the  spot.  Along  the  whole 
length  of  the  spot-spectrum  the  background  is  darkened, 
showing  a  general  absorption  ;  and  in  the  upper  part  of 
the  spectrum,  from  F  to  H,  this  seems  to  be  pretty 
much  all  that  can  be  noticed.  The  middle  portion  of 
the  spectrum,  however,  under  extremely  high  dispersion 
is  different  in  this  respect,  as  was  discovered  by  the 
writer  in  1883,  and  has  since  been  abundantly  confirmed 
by  Duner  and  others.  In  many  spots,  especially  large 
ones  that  are  nearly  circular  and  quiescent  with  a  very 
dark  nucleus,  the  spectrum  of  the  nucleus  between  E 
and  F  is  not  continuous,  but  is  made  up  of  countless 
fine,  dark  lines,  for  the  most  part  touching  or  slightly 
overlapping,  leaving  here  and  there,  however,  unoccu- 
pied intervals  which  look  like  (and  may  be)  bright  lines. 
Each  dark  line  is  spindle-shaped — i.  e.,  thicker  in  the 
middle  where  the  spectrum  is  darkest,  and  tapers  to  a 
fine,  faint,  hair-like  mark  at  each  end ;  most  of  them 
can  be  traced  across  the  penumbra-spectrum,  and  even 
out  upon  the  general  surface  of  the  sun.  The. 


SUN-SPOTS  AND  THE  SOLAR  SURFACE.  133 

age  distance  between  the  lines  is  about  half  that  be- 
tween the  two  components  of  58,  so  that  within  the  b 
group  the  total  number  of  dark  lines  is  some  300,  and 
there  are  seven  or  eight  of  the  bright  lines.  This 
structure  is  most  easily  seen  in  the  part  of  the  spectrum 
between  E  and  F;  above  F  the  lines  are  crowded  so 
closely  that  it  is  difficult  to  resolve  them,  and  below  E 
they  appear  to  grow  wider,  more  diffuse,  and  fainter.  It 
seems  to  indicate  that  the  principal  absorption  which 
darkens  the  center  of  a  sun-spot  is  not  such  as  would  be 
caused  by  minute  solid  or  liquid  particles — by  smoke  or 
cloud — which  would  give  a  continuous  spectrum  ;  but  it 
is  a  true  gaseous  absorption,  producing  a  veritable  dark- 
line  spectrum,  in  which  the  lines  are  countless  and  con- 
tiguous. 

In  the  lower  part  of  the  spectrum,  especially  between 
C  and  D,  the  spot-spectrum  is  full  of  interesting  details 
and  peculiarities,  which  deserve  a  far  more  thorough 
and  prolonged  study  than  they  have  yet  received. 
Many  of  the  dark  lines  of  the  ordinary  spectrum  are 
wholly  unmodified  in  the  spectrum  of  the  spot ;  in  fact, 
this  seems  to  be  the  case  with  the  majority  of  them. 
Others,  however,  are  much  widened  and  darkened,  and 
some,  which  are  hardly  visible  at  all  in  the  ordinary 
spectrum,  are  so  strong  and  black  as  to  be  very  conspicu- 
ous :  these  are  usually  spindle-shaped,  much  wider  in 
the  center  of  the  nucleus  than  at  its  edges  and  in  the 
penumbra,  so  that  they  are  often  called  "  fish-bellies." 
Certain  other  lines,  which  are  strong  in  the  ordinary 
spectrum,  thin  out  and  almost  disappear  in  the  spot- 
spectrum,  and  some  are  even  reversed  at  times.  There 
are  also  a  number  of  bright  lines,  not  very  brilliant,  to 
be  sure,  but  still  unmistakable,  and  there  are  some  dark 
shadings  of  peculiar  appearance. 


134 


THE   SUN. 


The  annexed  figure  (Fig.  35),  which  represents  a 
small  portion  of  the  spectrum  of  a  spot  observed  by  the 
writer  in  1872,  shows  nearly  all  of  these  peculiarities. 
The  portion  represented  lies  between  C  and  D,  the  scale 
attached  being  that  of  Kirchhoff's  map. 


FIG.  35. 
78    .     719 


i  J 


iil 


PORTION  OF  SUN-SPOT  SPECTRUM  BETWEEN  C  AND  D. 

Speaking  in  a  general  way,  the  lines  of  hydrogen, 
iron,  titanium,  calcium,  sodium,  and  vanadium  are  spe- 
cially affected.  The  hydrogen  lines  are  often  reversed; 
those  of  iron,  titanium,  calcium,  and  vanadium  are  usu- 
ally thickened,  and  those  of  sodium  are  often  enormously 
widened,  and  occasionally  both  widened  and  doubly 
reversed,  as  shown  in  Fig.  36,  which  represents  their 
appearance  in  the  spectrum  of  a  spot  observed  on  Sep- 
tember 22,  1870.  It  will  be  noticed  that  at  the  same 
time  the  helium-line,  D8,  which  usually  is  invisible  on 
the  solar  surface,  was  quite  conspicuous  as  a  dark  shade. 
On  this  occasion  the  lines  of  magnesium  also  behaved 
in  the  same  manner  as  those  of  sodium. 

As  has  already  been  mentioned  (page  109),  the  II 
and  K  bands  are  always  reversed  in  the  sun-spot  spec- 
trum. Usually,  over  the  spot  itself,  the  reversal  is 
only  "  single,"  but  double  reversal  is  not  very  uncommon. 


SUN-SPOTS   AND   THE   SOLAR  SURFACE. 


135 


Mr.  Lockyer  announces,  as  a  result  of  a  long  series  of 
observations,  that  there  is  a  striking  difference  between 
the  spot  spectra  at  the  time  of  maximum  and  minimum 
sun-spot  frequency  ;  the  lineb  that  are  most  conspicuous 
by  widening  and  darkening  being  by  no  means  the  same 
in  the  two  cases.  The  most  remarkable  change  is  in  the 
lines  of  iron,  which  are  usually  conspicuous,  but  almost 
vanish  from  the  spot-spectrum  at  the  sun-spot  maximum. 
At  times,  also,  the  spectrum  of  a  spot  gives  evidence 
of  violent  motion  in  the  overlying  gases  by  distortion 
and  displacement  of  the  lines.  When  the  phenomenon 


FIG.  36. 


m 


REVERSAL  OF  THE  D-LiNE8. 


occurs,  it  is  more  usually  at  points  near  the  outer  edge 
of  the  penumbra  than  over  the  central  portion  of  the 
spot ;  but,  occasionally,  the  whole  neighborhood  is  vio- 
lently agitated.  In  such  cases  it  often  happens  that 
lines  in  the  spectrum  side  by  side  are  affected  in  en- 
tirely different  ways — one  will  be  greatly  displaced, 
while  its  neighbor  is  not  disturbed  in  the  least,  showing 
that  the  vapors  which  produce  the  lines  are  at  different 
levels  in  the  solar  atmosphere,  and  do  not  participate 
to  any  great  extent  in  each  other's  movements. 

It  is  an  important  fact  that  the  same  thing  is  often 
true  of  lines  which  are  ascribed  to  a  single  substance : 
of  two  iron  lines,  for  instance,  one  may  be  disturbed 


136  THE  SUN. 

and  another  unaffected.  Mr.  Lockyer  lays  great  stress 
on  this  as  supporting  his  dissociation  hypothesis ;  but 
other  explanations  are  also  available,  see  page  91. 

In  a  few  instances  the  gaseous  eruptions  in  the 
neighborhood  of  a  spot  are  so  powerful  and  brilliant 
that,  with  the  spectroscope,  their  forms  can  be  made 
out  on  the  background  of  the  solar  surface  in  the  same 
way  that  the  prominences  are  seen  at  the  edge  of  the 
sun.  In  fact,  there  is  probably  no  difference  at  all  in 
the  phenomena,  except  that  only  prominences  of  most 
unusual  brightness  can  thus  be  detected  on  the  solar 
surface.  An  occurrence  of  this  kind  fell  under  the 
writer's  observation  on  September  28, 1870.  A  large 
spot  showed  in  the  spectrum  of  its  umbra  all  the  lines 
of  hydrogen,  magnesium,  sodium,  and  some  others,  re- 
versed. Suddenly  the  hydrogen  lines  grew  greatly 
brighter,  so  that,  on  opening  the  slit  of  the  spectroscope, 
two  immense  luminous  clouds  could  be  made  out,  one  of 
them  nearly  130,000  miles  in  length,  by  some  20,000  in 
width,  the  other  about  half  as  long.  They  seemed  to 
issue  at  one  extremity  from  two  points  near  the  edge 
of  the  penumbra  of  the  spot,  After  remaining  visible 
about  twenty  minutes,  they  faded  gradually  away,  with- 
out apparent  motion 

In  addition  to  spots,  sucn  as  we  nave  been  dealing 
with,  there  are  occasionally  seen  on  the  solar  surface 
dark-gray  patches,  which  Trouvelot,  who  first  called 
attention  to  them  in  1875,  has  named  "  veiled  spots,"  * 
considering  that  they  are  essentially  of  the  same  nature 
as  other  spots,  but  differing  in  this,  that  the  disturb- 
ance which  generates  them  is  not  sufficiently  powerful 
to  reach  the  surface  and  break  entirely  through  the 

*  For  Trouvelot's  account  of  them,  see  "  American  Journal  of  Science 
and  Art,"  March,  1876,  Third  Series,  vol.  xi. 


SUN-SPOTS   AND  THE  SOLAR   SURFACE.  137 

photosphere.  Over  these  veiled  spots  the  bright  gran- 
ules are  less  numerous  and  smaller  than  elsewhere,  but 
much  more  mobile ;  sometimes,  and  frequently  indeed, 
they  are  overlaid  by  faculae.  The  changes  of  form  and 
appearance  in  these  objects  are  very  rapid,  affairs  of  a 
minute  or  two  only,  according  to  Trouvelot.  They  are 
found  all  over  the  solar  surface,  not  being  at  all  con- 
fined to  the  regions  occupied  by  the  ordinary  spots, 
but  sometimes  occurring  within  eight  or  ten  degrees  of 
the  sun's  pole.  They  have  been  little  observed,  how- 
ever, and  information  respecting  them  is  as  yet  very 
meager. 

ROTATION   OF   SUN   AND    PROPER   MOTIONS    OF    SPOTS. 

We  have  already  mentioned  that  the  spots  travel 
across  the  disk  of  the  sun,  from  the  eastern  edge  to 
the  western,  in  such  a  manner  as  to  show  that  they  arc 
attached  to  the  surface,  and  that  the  sun  rotates  upon 
its  axis.  The  true  period  is  about  twenty-five  days,* 
the  apparent  or  "  synodic  "  period  being  some  two  days 
longer,  because  the  earth  itself  is  continually  moving 
forward  in  its  orbit. 

When  we  come,  however,  to  study  the  motions  of 
the  spots  more  carefully,  we  find  that  they  have  move- 
ments of  their  own  (proper  motions,  as  astronomers  call 
them),  both  in  latitude  and  longitude,  so  that  no  observa- 
tions of  any  single  spot,  however  carefully  conducted, 

*  It  is  perhaps  worth  noting  that,  between  the  sun  and  the  earth's 
magnetism,  there  is  an  unquestionable,  though  still  unexplained,  connec- 
tion, which  shows  itself  in  many  ways.  Among  the  numerous  periodic 
variations  of  this  magnetism  Hornstein  finds  one  with  a  period  of  26'32 
days.  Assuming  this  to  be  due  to  the  sun's  synodic  rotation,  he  gets 
24-55  days  for  the  true  rotation.  Very  similarly  Bigelow  deduces  24'86. 
Veeder's  "  aurora  period  "  (27'28  days)  gives  25'38— all  of  which  may 
be  taken  for  what  it  is  worth. 


138  THE  SUN. 

can  furnish  an  accurate  determination  of  the  position  of 
the  sun's  axis  and  its  period  of  rotation.  This  fact  does 
not  seem  to  have  been  comprehended  by  the  early  ob- 
servers (though  a  neglected  remark  of  Schemer's  indi- 
cates that  he  had  a  glimpse  of  the  truth),  and  hence  we 
have  serious  discordances  between  their  different  results, 
which  range  from  25'01  days,  the  result  obtained  by 
Delambre  in  1775,  to  25*58  days,  as  determined  by 
Cassini  about  a  hundred  years  earlier.  The  different 
values  for  the  inclination  of  the  sun's  equator  to  the 
ecliptic  lie  between  6-J-0  and  7£°,  and  those  for  the  lon- 
gitude of  the  node  between  70°  and  80°.  The  most 
reliable  recent  results  are  those  of  Carrington  and 
Spoerer.  The  former  makes  the  mean  period  of  the 
sun's  rotation  25*38  days,  while  Spoerer  gives  it  as 
25*23. 

THE   EQUATORIAL   ACCELERATION. 

The  researches  of  Carrington,*  between  1853  and 
1861,  first  brought  out  clearly  the  fact  that,  strictly 
speaking,  the  sun,  as  a  whole,  has  no  single  period  of 
rotation,  but  different  portions  of  its  surface  perform 
their  revolutions  in  different  times.  The  equatorial  re- 
gions not  only  move  more  rapidly  in  miles  per  hour 
than  the  rest  of  the  solar  surface,  but  they  complete  the 
entire  rotation  in  shorter  time.  If  we  deduce  the 
period  by  means  of  spots  near  the  sun's  equator,  we 
shall  find  it  to  be  very  nearly  25  days,  a  trifle  less — 
according  to  Carrington.  Spots  at  a  solar  latitude  of 

*  A  memoir  by  Laugier,  presented  to  the  French  Academy  in  1844, 
but  never  published  in  eztenso,  contains,  according  to  Faye,  data  which 
would  lead  to  the  same  result.  The  summary,  given  in  the  "  Comtes 
Rendus,"  fails,  however,  to  indicate  any  appreciation  of  the  systematic 
variation  of  rotation  rate  from  equator  to  poles,  and  in  no  way  invali- 
dates Carrington' s  claim  to  be  considered  the  discoverer  of  the  law. 


SUN-SPOTS  AND  THE  SOLAR  SURFACE.  139 

20°  have,  on  the  other  hand,  a  period  nearly  18  hours 
longer ;  at  30°  the  period  rises  to  26£  days,  and  at  45° 
to  2TJ,  though  in  this  latitude  there  are  so  few  spots 
that  the  determination  is  not  very  reliable.  Beyond 
this  latitude  we  have  nothing  satisfactory,  and  it  is  not 
possible  to  determine,  with  any  certainty,  whether  this 
retardation  continues  to  the  pole  or  not. 

It  is  a  curious  circumstance,  probably  connected 
with  this  remarkable  law  of  surface-movement,  that  the 
spots  mostly  lie  between  ten  and  thirty-five  degrees 
of  latitude  on  each  side  of  the  sun's  equator ;  and  it  is 
this  fact  which  makes  it  difficult  to  ascertain  the  exact 
laws  of  the  solar  rotation,  since  our  observations  are 
confined  to  such  a  limited  range  of  latitude.  As  yet, 
no  points  have  been  found  near  the  sun's  poles  perma- 
nent and  definite  enough  to  permit  precise  observations 
covering  a  sufficient  interval  of  time. 

By  a  discussion  of  all  his  observations,  more  than 
5,000  in  number,  of  954  different  groups  of  spots,  Mr. 
Carrington  deduced  the  expression  X  —  865'— 165'  sin^ 
for  the  daily  motion  of  the  surface  of  the  sun  in  dif- 
ferent solar  latitudes,  I  representing  the  latitude  in  the 
formula,  and  X  the  daily  motion  in  minutes  of  solar 
longitude.  This,  as  was  said  before,  would  make  the 
rotation  period  of  the  sun's  equator  a  little  less  than 
25  days.  The  expression,  however,  is  purely  empirical, 
and  no  imaginable  theoretical  explanation  can  be  given 
for  the  fractional  exponent  J. 

Faye,  assuming  on  theoretical  grounds  that  this  ex- 
ponent ought  to  be  2,  finds  from  the  same  observations 
the  formula  X  =  862'—  186'  sinV,  an  expression  which 
agrees  with  all  but  a  few  of  the  observations  nearly  as 
well  as  Carrington's. 

Spoerer,  from  observations  of  his  own,  made  be- 


140  THE  SUN. 

tween  1862  and  1868,  and  combined  with  those  of 
Secchi  and  others,  derives  the  still  different  formula, 
X  =  1011'  -  203'  sin(41°  13'+ 1). 

Tisserand,  from  observations  of  325  spots  in  1874- 
'75,  deduces  the  expression  X  =  857'*6  —  157''3  sin'Z. 
But  this  is  probably  less  reliable  than  either  of  the  pre- 
ceding, being  founded  on  a  much  smaller  number  of  ob- 
servations. 

Wilsing,  of  Potsdam,  in  1888  published  a  discussion 
of  several  hundred  faculce  shown  on  their  phothelio- 
graph  plates,  and  deduced  a  rotation-period  of  25*23 
days  ;  but  he  found  no  indications  of  equatorial  acceler- 
ation, and  concluded  that  this  peculiarity  of  the  photo- 
sphere, where  the  spots  have  their  residence,  does  not 
extend  to  the  region  of  the  faculse — a  very  perplexing 
fact,  if  real.  Still  more  recently,  however,  Stratonoff,  of 
Pulkowa,  from  a  discussion  of  their  plates,  finds  from 
the  faculse  a  result  quite  in  accordance  with  those  of 
Carrington  and  Spoerer. 

We  have  already  referred  to  the  evidence  of  the 
sun's  rotation  given  by  the  spectroscope  (page  100), 
and  have  specially  quoted  the  remarkable  work  of 
Duner.  His  results  show  (we  think  conclusively, 
though  objections  have  been  raised  in  certain  quar- 
ters) that  the  region  in  which  the  dark  lines  of  the 
spectrum  originate  share  perfectly  the  motion  of  the 
photosphere.  His  observations,  moreover,  have  this 
great  advantage  over  those  made  on  spots  and  faculae, 
that  they  extend  as  far  as  75°  on  each  side  of  the  sun's 
equator.  The  observations  are  very  well  represented  by 
the  equation  X  =  846'—  272'*4  sin-7.  This  would  cor- 
respond to  a  rotation-period  of  25*53  days  at  the  sun's 
equator  and  about  37*5  at  the  pole — but  the  polar 
period  is  very  uncertain. 


SUN-SPOTS  AND  THE  SOLAR  SURFACE. 

While  either  of  the  formulse  given  above  agrees 
fairly  with  the  facts  observed,  neither  of  them  can  be 
regarded  as  logically  established  upon  a  sound  physical 
explanation.* 

The  cause  of  this  peculiar  surface-drift  is  not  yet 
known.  Sir  John  Herschel  was  disposed  to  attribute 
it  to  the  impact  of  meteoric  matter  striking  the  sun's 
surface  mainly  in  the  neighborhood  of  the  equator,  and 
so  continually  accelerating  its  rotation,  as  a  boy's  peg- 
top  is  whipped  up  by  the  skillfully  applied  lash.  Per- 
haps there  is  nothing  absurd  in  the  idea  that  a  sufficient 
quantity  of  meteoric  matter  may  reach  the  sun,  or  that 
the  meteors  move,  for  the  most  part,  in  the  plane  of 
the  sun's  equator,  and  direct,  i.  e.,  with  and  not  against 
the  motion  of  the  planets — so  that  their  fall  would  be 
mostly  confined  to  the  equatorial  regions,  and  would 
thus  hasten,  and  not  retard,  the  surface  motion. 

But  then  the  duration  of  the  sun's  rotation  period 
should  continually  grow  shorter,  an  effect  which  does 
not  appear  from  a  comparison  of  Scheiner's  results  with 
those  most  recently  obtained.  Of  course,  it  may  be 
that  such  an  acceleration  has  actually  occurred,  only  too 
small  to  be  yet  detected  ;  still,  it  would  seem  probable 
that  any  "driving,"  sufficient  to  establish  nearly  two 
days'  difference  between  the  rotation  periods  at  the 
equator  and  at  latitude  40°,  must  have  produced  a  very 
sensible  effect  within  three  hundred  years. 

It  is  more  probable  that  the  equatorial  acceleration 
is  connected  in  some  way  with  the  exchange  of  matter 
which,  if  the  sun  is  for  the  most  part  gaseous,  as  now 
seems  likely,  must  continually  be  going  on  between  the 
outside  and  inside  of  the  globe.  If  the  photosphere  is 
formed  of  masses  falling,  such  an  effect  would  be  a 
necessary  consequence.  If  we  suppose  that  the  out- 

*  See  Note  E,  page  360. 


142  THE  SUN- 

rushing  streams  of  heated  gas  and  vapor,  as  they  rise, 
continue  in  the  gaseous  condition  until  they  reach  the 
summit  of  their  ascent,  and  remain  at  this  height  long 
enough  to  acquire  sensibly  the  rotation  velocity  corre- 
sponding to  their  altitude,  and  that  then  the  products  of 
condensation,  resulting  from  their  cooling,  fall  down- 
ward, and  thus  falling  constitute  the  photosphere,  we 
should  have  precisely  the  actual  phenomenon.  The  ro- 
tation velocity  of  each  visible  element  of  the  photosphere 
would  be  that  corresponding  to  a  greater  altitude,  and 
therefore  greater  than  that  naturally  belonging  to  its 
observed  position,  and  this  difference  would  vary  from 
the  equator,  where  it  would  be  a  maximum,  to  the  poles, 
where  it  would  vanish. 

Of  course,  it  is  not  necessary  to  such  an  effect  that 
the  conditions  supposed  should  be  rigidly  complied 
with ;  it  will  suffice  to  admit  that  in  the  photosphere 
the  falling  masses  are  more  conspicuous  than  those 
which  are  ascending  or  stationary,  and  it  would  seem 
hardly  possible  that  it  should  be  otherwise.  Whether, 
however,  the  effect  thus  produced  would  account  in 
measure  as  well  as  kind  for  the  observed  phenomena, 
is  a  question  requiring  for  its  answer  a  more  thorough 
mathematical  investigation  than  the  writer  has  yet  been 
able  to  undertake. 

If  we  consider  only  the  spots,  it  would  seem  entirely 
possible  that  they  may  be  produced  by  matter  which 
has  fallen  from  a  height  of  even  fifteen  or  twenty  thou- 
sand miles,  and  that  fall  would  be  quite  sufficient  to  ac- 
count for  their  whole  acceleration. 

The  fact  that  rapid  changes  in  the  configuration  of 
a  spot  are  generally  accompanied  by  an  eastward  rush 
of  the  whole,  also  favors  the  idea  that  a  downfall  of 
something  from  above  is  concerned  in  the  matter. 


SUN-SPOTS   AND   THE   SOLAR   SURFACE.  143 

If  we  rightly  understand  the  matter,  this  theory  of 
the  equatorial  acceleration  is  in  substantial  accordance, 
so  far  as  it  goes,  with  that  formulated  some  years  later 
by  Mr.  Lockyer,  and  given  in  the  last  chapter  of  his 
"  Chemistry  of  the  Sun."  But  his  "  dissociation  theory  " 
apparently  has  an  important  role  to  play  in  providing 
the  "  hundreds  of  millions  of  tons "  of  falling  matter 
that  produce  the  phenomena  by  their  "down  rush." 
Schaeberle  also  attributes  the  equatorial  acceleration  to 
the  falling  back  of  material  that  has  been  projected  to 
a  great  elevation  above  the  photosphere. 

The  idea  of  Faye  appears  to  have  been  nearly  the 
reverse  of  that  here  suggested.  He  attributes  the  for- 
mation of  the  photosphere  to  gaseous  matter  not  falling 
from  above,  but  ascending  from  below,  and  starting 
from  a  stratum  at  a  certain  depth  below  the  surface ; 
by  supposing  the  depth  of  this  stratum  to  vary  with 
the  latitudes,  being  greatest  at  the  poles  of  the  sun  and 
least  at  the  equator,  it  is  easy  to  explain  on  this  hy- 
pothesis the  accelerated  motion  of  the  surface  at  the 
equator,  and  to  justify  his  formula,  which  makes  the 
retardation  at  higher  latitudes  proportional  to  the  square 
of  the  sine  of  the  latitude  ;  but  no  reason  is  evident  why 
the  depth  of  this  stratum  should  vary. 

Certain  later  investigations  in  1886  upon  the  rota- 
tion of  fluid  masses,  by  Jukowsky,  of  Moscow,  as  ap- 
plied to  solar  conditions  by  his  colleague  Belopolsky, 
perhaps  warrant  a  hope  that  the  phenomena  of  surface- 
drift  in  longitude,  and  even  the  periodicity  of  the  spots, 
ultimately  find  a  rational  explanation  as  necessary  re- 
sults of  the  slow  contraction  of  a  non-homogeneous  and 
mainly  gaseous  globe.  The  subject  is  difficult  and 
obscure ;  but  if  it  can  be  proved,  as  seems  not  impossi- 
ble, that,  on  mechanical  principles,  the  time  of  rotation 


144  THE  SUN. 

of  the  central  portions  of  such  a  whirling  mass  must 
be  shorter  than  that  of  the  exterior,  then  there  will  be, 
of  necessity,  an  interchange  of  matter  between  the 
inside  and  outside  of  the  sphere,  a  slow  surface- 
drift  from  equator  toward  the  poles,  a  more  rapid  inter- 
nal current  along  and  near  the  axis,  from  the  poles 
toward  the  equator,  a  continual  "  boiling  up  "  of  internal 
matter  on  each  side  of  the  equator,  and,  finally,  just 
such  an  eastward  drift  near  the  equator  as  is  actually 
observed.  Moreover,  the  form  of  the  mass,  and  the 
intensity  of  the  drift  and  consequent  "  boiling  up  "  from 
underneath  might,  and  probably  would,  be  subject  to 
great  periodical  variations. 

As  to  Zollner's  idea  that  the  equatorial  acceleration 
is  due  to  the  friction  between  a  liquid  sheet,  constitut- 
ing the  photosphere,  and  a  solid  nucleus  below,  it  is 
hardly  necessary  to  say  that  this  view  is  in  complete 
opposition  to  those  held  by  almost  all  astronomers,  and 
seems  to  be  untenable  in  its  fundamental  assumptions. 

On  the  whole,  however,  the  writer  sympathizes  with 
Duner  in  his  conclusion  :  "  I  must  confess  that  this  dif- 
ference between  the  rotation  periods  in  the  different 
(solar)  latitudes  appears  to  me  incomprehensible,  and 
constitutes  one  of  the  most  difficult  problems  of  astro- 
physics." No  theory  yet  presented  is  really  satisfactory* 


The  plane  of  the  sun's  rotation  is  slightly  inclined 
to  that  of  the  earth's  orbit.  According  to  Carrington, 
the  angle  is  7°  15',  while  Spoerer  makes  it  6°  57'.  This 
plane  cuts  the  ecliptic  at  two  opposite  points  called  the 
nodes,  one  of  which  is  in  longitude  73°  40',  according  to 
Carrington,  and  74°  36'  according  to  Spoerer.  The 
axis  of  the  sun  is  therefore  directed  to  a  point  in  the 

*  See  Note  E,  page  360. 


SUN-SPOTS  AND   THE  SOLAR  SURFACE.  145 

constellation  of  Draco,  not  marked  by  any  conspicuous 
star.  Astronomers  define  its  position  by  saying  that 
its  right  ascension  is  18h  44m,  and  its  declination  is  64°. 
It  is  almost  exactly  half-way  between  the  bright  star 
a  Lyrse  and  the  polar  star. 

The  earth  passes  through  the  two  nodes  on  or  about 
the  3d  of  June  and  the  5th  of  December.  At  these 
times  the  spots  move  apparently  in  straight  lines  across 
the  sun's  disk,  and  his  poles  are  situated  on  its  circum- 
ference. During  the  summer  and  autumn,  from  June 
to  December,  the  sun's  northern  pole  is  inclined  toward 
the  earth  ;  during  the  winter  months,  the  southern.  The 
angle  which  the  sun's  axis  appears  to  make  with  a  north 
and  south  line  in  the  sky  (technically,  the  position-angle 
of  the  sun's  axis)  changes  considerably  during  the  year, 
varying  26°  each  side  of  zero.  As  it  is  often  very 
desirable  for  an  amateur  to  know  this  angle  approxi- 
mately, we  insert  the  following  little  table,  giving  the 
position  angle  of  the  sun's  north  pole  referred  to  the 
center  of  the  disk.  The  table  is  derived  from  the  much 
more  extensive  one  in  Secchi's  "  Le  Soleil "  : 

POSITION  ANGLE   OF  SUN'S  AXIS. 
JANUARY  4,  JULY  6 0°-00. 


Jan.  15,  June  25.... 
Jan.  26,  June  14.  ... 
Feb.  7,  June  2. 

5°  west. 
10°  west. 
15°  west. 

Dec.  24,  July  17... 
Dec.  15,  July  29... 
Dec.  3,  Aug.  11  

1    5°  east. 
10°  east. 
15°  east. 

Feb.  22,  May  18  
March  18,  April  25.. 
April  5  

20°  west. 
25°  west. 
26*  20'  west. 

Nov.  19,  Aug.  27... 
Oct.  29,  Sept.  20.  .  . 
Oct.  10  

20°  east. 
'  25°  east. 
26°  20'  east. 

It  is  understood,  of  course,  that  the  table  is  only 
approximate,  because  the  numbers  change  slightly  ac- 
cording to  the  place  of  the  current  year  in  the  leap- 
year  cycle ;  but  the  results  obtained  from  it  are  always 
11 


146 


THE  SUN. 


correct  within  about  J°,  which  is  near  enough  for  most 
purposes. 

Fig.  37  illustrates  these  points,  giving  the  position- 
angle  of  the  sun's  axis,  and  the  aspect  of  his  equator  at 
different  times  of  the  year  as  seen  from  the  earth.  For 


FIG.  87. 


APRIL  6, 


N.  261?  W. 


OCT.  10,  N.  2B*?  E. 


SEPT.  4.*  DEC.  5.  MARCH*  7.* 

POSITION-ANGLE  OF  SUN'S  Axis,  AND  ASPECT  OF  HIS  EQITATOB. 

the  sake  of  clearness,  however,  the  inclination  of  the 
sun's  equator  to  the  ecliptic  is  considerably  exaggerated 
in  the  lower  row  of  figures :  the  equator  never  appears 
so  strongly  curved  as  there  represented. 

PROPER   MOTION   OF    SPOTS. 

After  making  due  allowance  for  the  equatorial 
acceleration,  it  is  found  that  almost  every  spot  has  more 
or  less  motion  of  its  own.  Between  latitudes  20°  north 
and  20°  south,  Mr.  Carrington  finds,  on  the  whole,  a 
slight  tendency  to  motion  toward  the  equator,  the  move- 
ment amounting  to  a  minute  or  two  of  arc  per  diem ; 
from  20°  to  30°  on  both  sides  of  the  equator,  there  ie 


SUN-SPOTS  AND   THE  SOLAR  SURFACE.  14.7 

a  somewhat  more  decided  motion  toward  the  poles. 
Faye  has  also  shown  that  many  spots  move  in  small 
ellipses  upon  the  surface  of  the  sun,  completing  their 
circuits  in  a  day  or  two,  and  repeating  them  with  great 
regularity  for  weeks,  and  even  months.  Whenever  a 
spot  is  passing  through  sudden  changes,  it  generally 
moves  forward  upon  the  solar  surface,  as  has  already 
been  mentioned,  with  something  like  a  leap ;  and,  when 
a  spot  divides  into  two  or  more,  the  parts  generally  sep- 
arate with  a  very  considerable  velocity,  as  if  (we  do  not 
say  because)  there  was  a  repulsion  between  them. 

DISTRIBUTION   OF    SUN-SPOTS. 

The  sun-spots,  as  has  already  been  said,  are  not  dis- 
tributed over  the  sun's  surface  with  anything  like  uni- 
formity. They  occur  mainly  in  two  zones  on  each  side 
of  the  equator  and  between  the  latitudes  of  10°  and  30°. 
On  the  equator  itself  they  are  comparatively  rare ;  there 
are  still  fewer  beyond  35°  of  latitude,  and  only  a  single 
spot  has  ever  been  recorded  more  than  4:5°  from  the 
solar  equator — one  observed  in  1846  by  the  late  Dr. 
Peters,  then  in  Naples. 

The  figure  shows  the  distribution  of  1,386  spots  ob- 
served by  Carrington.  The  figure  is  constructed  in 
this  way :  The  circumference  of  the  sun,  on  the  left- 
hand  side  of  the  figure,  is  divided  into  five-degree 
spaces  from  the  equator  each  way,  and  at  each  of  them 
is  erected  a  radial  line  whose  length  in  four  hundredths 
of  an  inch  is  proportional  to  the  number  of  spots  ob- 
served within  2£°  of  latitude  on  each  side.  Thus,  the 
line  drawn  at  20°  north  latitude,  and  marked  "  151,"  is 
J£j-  of  an  inch  long,  and  means  that  151  spots  were 
recorded  between  17^°  and  22£°  north  latitude. 

It  is  at  once  evident  from  mere  inspection  that  the 


148 


THE  SUN. 


distribution  follows  no  simple  law  of  latitude.  On  the 
northern  hemisphere,  the  distribution,  during  the  eight 
years  over  which  the  observations  extend,  was  not  very 
irregular,  though  there  is  a  distinct  minimum  at  15°, 


FIG.  38. 


DISTRIBUTION  OF  SUN-SPOTS  AND  PBOTUBEBANCM. 

and  two  maxima  at  about  11°  and  22°  of  latitude.  On 
the  southern  hemisphere  the  minimum  at  15°  is  very 
marked,  and  the  numbers  at  10°  and  20°  are  far  in 
excess  of  those  in  the  northern  hemisphere.  Of  the 
whole  number,  711  were  in  the  southern  hemisphere, 
as  against  675  in  the  northern. 

The  minimum  at  15°  of  latitude  was  special  to  the 
date  of  observation,  and  had  its  origin  in  a  law  discov- 
ered by  Spoerer  a  few  years  ago — to  be  discussed  later 
(page  156).  His  own  observations  from  1861  to  1867 
show  nothing  of  the  kind.  They  give  the  following 


SUN-SPOTS  AND   THE  SOLAR   SURFACE.  149 

distribution  of  1,053  spots  in  latitude,  viz. :  -f-  35°,  4  ; 
+  30°,  4 ;  +  25°,  16  ;  +  20°,  50 ;  +15°,  133  ;  + 10°, 
198;  +5°,  114— in  all,  519  spots  north  of  the  solar 
equator.  40  spots  were  on  the  equator,  or  within  2°  of 
it.  South  of  the  equator  we  have,  in  latitude :  —  5°, 
113 ;  -  10°,  206  ;  -  15°,  109  ;  -  20°,  38 ;  -  25°,  19  ; 
-  30°,  7 ;  -  35°,  1 ;  -  40°,  1— in  all,  494  southern 
spots.  In  1866,  a  year  of  spot  minimum,  there  were 
only  94  spots  in  all,  and  of  these  94  all  but  two  were 
situated  within  17°  of  the  equator. 

It  is  to  be  noticed  that  at  times  when  spots  are 
abundant  their  mean  latitude  is  greater  than  when  they 
are  few,  or,  in  other  words,  an  increase  in  the  number 
of  spots  generally  carries  with  it  a  widening  of  the  zones 
in  which  the  spots  appear.  All  the  observations  concur 
in  showing  this. 

The  cause  of  this  distribution  of  the  spots  in  zones 
is  not  known.  It  is  probably  connected  with  the  origin 
of  the  spots  themselves,  and  very  possibly  has  something 
to  do  with  the  law  of  surface-motion  just  discussed.  At 
least  it  is  certain,  as  Faye  pointed  out  some  years  ago, 
that,  while  at  the  solar  poles  and  equator  adjoining  por- 
tions of  the  photosphere  have  no  relative  motion  with 
reference  to  each  other,  yet  in  the  middle  latitudes 
this  is  not  true ;  here  each  element  of  the  surface  has  a 
different  velocity  from  those  immediately  north  and 
south  of  it,  so  that  they  drift  by  each  other  like  the 
filaments  of  a  liquid  current  which  is  suffering  retarda- 
tion, producing,  as  Faye  supposes,  whirlpools  and  eddies 
which,  according  to  his  view,  generate  the  spots. 

As  regards  the  sun's  northern  and  southern  hemi- 
spheres, there  is  often  a  great  inequality.  Thus,  from 
1672  to  1704  absolutely  no  spots  were  recorded  in  the 
northern  hemisphere,  and  when  a  few  appeared  in  1705 


150  THE  SUN. 

and  1714:,  the  French  Academy  was  formally  notified  of 
the  fact  as  something  very  remarkable.  We  do  not 
know  that  anything  quite  like  this  has  ever  happened 
since ;  but  the  inequality  between  the  two  hemispheres 
is  often  very  marked  for  months  together,  though  in 
the  long  run  there  seems  to  be  no  difference. 

It  is  a  question  of  much  theoretical  importance 
whether  spots  do  or  do  not  appear  repeatedly  at  the 
same  points;  for  if  this  is  really  the  case,  it  would 
make  it  almost  certain  that  below  the  photosphere  there 
must  be  a  coherent  nucleus,  carrying  with  it  in  its  rota- 
tion such  volcanic  or  otherwise  peculiar  regions  as  to 
cause  the  breaking  out  of  spots  above  them.  There 
would  be  no  difficulty  in  accounting  for  two  or  three 
dissolutions  and  reappearances  in  the  same  region  with- 
out any  such  hypothesis,  since  a  great  disturbance  in 
the  solar  atmosphere  would  not  subside  entirely  for  a 
long  time.  The  observations  of  Spoerer  show  that 
this  actually  happens,  and  that,  for  a  period  of  several 
months,  spots  and  faculse  often  recur  several  times  at 
the  same  point.  But  his  observations  do  not  give  any 
real  support  to  the  idea  of  a  solid  nucleus,  nor  has  he 
himself  ever  favored  such  a  view,  although  some  (and 
among  others  the  writer),  misunderstanding  certain  ex- 
pressions of  his,  have  supposed  that  he  did. 


CHAPTER  V. 

PERIODICITY  OF  SUN-SPOTS;    THEIR  EFFECTS  UPON  THE  EARTH. 
AND   THEORIES  AS  TO  THEIR   CAUSE  AND  NATURE. 

Observations  of  Schwabe. — Wolf's  Numbers. — Proposed  Explanations  of 
Periodicity. — Connection  between  Sun-Spots  and  Terrestrial  Magnet- 
ism.— Remarkable  Solar  Disturbances  and  Magnetic  Storma. — Effect 
of  Sun-Spots  on  Temperature. — Sun-Spots,  Cyclones,  and  Rainfall. — 
Researches  of  Symons  and  Meldrum. — Sun-Spots  and  Commercial 
Crises. — Galileo's  Theory  of  Spots. — Herschel's  Theory. — Secchi's 
First  Theory. — Zollner's. — Faye's. — Secchi's  Later  Opinions. — Theo- 
ries of  Lockyer,  Schaeberle,  and  others. 

IT  was  early  noticed  that  the  number  of  sun-spots  is 
very  variable,  but  the  discovery  of  a  regular  periodicity 
in  their  number  dates  from  1851,  when  Schwabe,  of 
Dessau,  first  published  the  result  of  twenty-five  years 
of  observation.  During  this  time  he  had  examined  the 
sun  on  every  clear  day,  and  had  secured  an  almost  per- 
fect record  of  every  spot  that  appeared  upon  the  solar 
surface.  He  began  his  work  without  any  idea  of  ob- 
taining the  result  he  arrived  at,  and  says  of  himself, 
that,  "  like  Saul,  he  went  to  seek  his  father's  asses,  and 
found  a  kingdom."  His  observations  showed  unmis- 
takably that  there  is  a  pretty  regular  increase  and  de- 
crease in  the  number  of  sun-spots,  the  interval  from  one 
maximum  to  the  next  being  not  far  from  ten  years. 
Subsequent  observations  and  a  thorough  examination 
of  all  known  former  records  fully  confirm  this  conclu- 


152  THE   SUN. 

sion,  except  that  the  mean  period  appears  to  be  some- 
what greater,  eleven  and  one  ninth  years  being  the  value 
at  present  generally  received.  Professor  K.  Wolf,  of 
Zurich,  has  been  especially  indefatigable  in  his  investi- 
gations upon  this  subject,  and  has  succeeded  in  disinter- 
ring from  all  sorts  of  hiding-places  a  nearly  complete 
history  of  the  solar  surface  for  the  past  hundred  and 
fifty  years.  Among  other  things  he  finds  among  the 
unpublished  manuscripts  of  Horrebow  (a  Danish  as- 
tronomer who  flourished  a  century  ago)  a  distinct  in- 
timation (in  1776)  that  zealous  and  continued  observa- 
tion of  the  sun-spots  might  lead  to  "  the  discovery  of  a 
period,  as  in  the  motions  of  the  other  heavenly  bodies," 
with  the  added  remark  that  "  then,  and  not  till  then, 
it  will  be  time  to  inquire  in  what  manner  the  bodies 
which  are  ruled  and  illuminated  by  the  sun  are  influ- 
enced by  the  sun-spots  " — alluding,  perhaps,  to  certain 
ideas  then,  as  now,  more  or  less  current,  and  illustrated 
by  the  attempt  of  Sir  W.  Herschel,  a  few  years  later,  to 
establish  a  relation  between  the  price  of  wheat  and  the 
number  of  sun-spots. 

Wolf  has  brought  together  an  enormous  number  of 
observations,  and  with  immense  labor  has  combined 
them  into  a  consistent  whole,  deducing  a  series  of  "  rel- 
ative numbers,"  as  he  calls  them,  which  represent  the 
state  of  the  sun  as  to  spottedness  for  every  year  since 
1745.  His  "relative  number"  is  formed  in  rather  an 
arbitrary  manner  from  the  observation  of  the  spots : 
representing  this  number  by  r,  the  formula  is,  r  = 
k(f-{- 10<7),  in  which  y  is  the  number  of  groups  and 
isolated  spots  observed,  and  f  the  total  number  of 
spots  which  can  be  counted  in  these  groups  and  singly, 
while  It  is  a  coefficient  which  depends  upon  the  ob- 
server and  his  telescope.  Wolf  takes  it  as  unity  for 


PERTODIClfY   OF  SUN-SPOTS. 


153 


154  THE  SUN. 

himself,  observing  with  a  three-inch  telescope  and  power 
of  64.  For  an  observer  with  a  larger  instrument,  k  would 
be  a  smaller  quantity,  while  a  less  powerful  instrument 
and  less  assiduous  observer  would  receive  a  "&"  greater 
than  unity,  as  probably  seeing  fewer  spots  than  Wolf 
himself  would  reach  with  his  instrument.  These  rela- 
tive numbers,. as  tested  by  the  most  recent  photographic 
results  of  De  La  Rue  and  Stewart,  are  found  to  be  quite 
approximately  proportional  to  the  area  covered  by  the 
spots. 

We  give  on  the  opposite  page  a  figure  deduced  from 
the  numbers,  published  by  Wolf  in  1877,  in  the  "Me- 
moirs of  the  Royal  Astronomical  Society,"  and  showing 
their  course  year  by  year  since  1772.  The  continua- 
tion* of  the  curve  to  1880  is  from  numbers  subse- 
quently published  by  him  in  the  astronomical  periodi- 
cals. The  horizontal  divisions  denote  years,  and  the 
height  of  the  curve  at  each  point  gives  the  "  relative 
number"  for  the  date  in  question.  For  example,  in 
1870,  about  the  middle  of  the  year,  the  relative  num- 
ber was  140,  while  early  in  1879  it  ran  as  low  as  3. 

The  dotted  lines  are  curves  of  magnetic  disturbance, 
with  which  at  present  we  have  no  concern.  Our  dia- 
gram, on  account  of  the  smallness  of  the  page,  only  goes 
back  to  1772,  but  Wolf's  investigations  reach  to  1610, 
and  he  gives,  in  the  paper  from  which  were  derived  the 
numbers  used  in  constructing  our  diagram,  the  follow- 
ing important  table  of  the  maxima  and  minima  of  sun- 
spots  since  that  date,  dividing  the  results  into  two  series, 
the  first  of  which,  from  the  paucity  of  observations,  is 
to  be  considered  of  much  inferior  weight  to  the  second. 

*  It  did  not  seem  worth  while  to  re-engrave  the  plate  in  order  to  bring 
the  curve  down  to  date,  but  the  main  results  since  1880  are  stated  nu- 
merically a  page  or  two  later. 


PERIODICITY   OF  SCTN-SPOTS. 


155 


FIRST  SERIES. 

SECOND  SBBIIS. 

Minima. 

Maxima. 

Minima. 

Maxims. 

1610-8 

1615-5 

1745-0 

1750-3 

8-2 

10.5 

10-2 

11-2 

1619-0 

1626-0 

1755-2 

1761-6 

15'0 

13-5 

11-3 

8-2 

1634-0 

1639-5 

1766-5 

1769-7 

11-0 

9-5 

9-0 

8-7 

1645-0 

1649-0 

1775-5 

1778-4 

10-0 

11-0 

92 

9-7 

1655-0 

1660-0 

1784-7 

1788-1 

11-0 

15-0 

13-6 

16-1 

1666-0 

1676-0 

1798-3 

1804-2 

13-5 

10-0 

12-3 

J2-2 

1679-5 

1685-0 

1810-6 

1816-4 

10-0 

8-0 

12-7 

13-5 

1689-5 

1693-0 

1823-3 

1829-9 

8'5 

12-5 

10-6 

7-3 

1698-0 

1705-5 

1833-9 

1837-2 

140 

12-7 

96 

10-9 

1712-0 

1718-2 

18435 

1848-1 

11-5 

9-3 

12-6 

120 

1723-5 

1727-5 

1856-0 

1860-1 

10-5 

11-2 

11-2 

10-5 

1734*0 

1738-7 

1867-2 

1870-6 

Mean  period. 

Mean  period. 

Mean  period. 

Mean  period. 

11-20±  2-11* 

11-20  ±  2-06 

11-16  ±  1-54 

10-94  ±  2-52 

±  0-64 

±0-63 

±0-47 

±0-76 

From  these  data,  Wolf  derives  a  mean  period  of 
11-111  years,  with  an  average  variability  of  2*03  years, 
and  an  uncertainty  of  0'307,  due  chiefly  to  the  difficulty 
of  fixing  the  precise  date  of  maximum  or  minimum. 

After  the  great  maximum  of  1871*6,  when  the  rel- 
ative number  reached  140,  there  was  an  unusually  pro- 
tracted down-slide  until  1879,  when,  as  the  figure  shows, 

*The  upper  number,  ±  2-11,  indicates  that  the  individual  periods 
have  an  average  variation  of  2*11  years  on  one  side  or  the  other  from  the 
mean  period.  The  lower  number,  ±  0'64,  is  the  so-called  "  probable 
error  "  of  the  period.  Similarly  in  the  three  other  columns. 


156  THE  SUN. 

a  very  low  minimum  occurred.  After  that  a  feeble 
maximum  (only  64)  arrived  pretty  quickly  near  the  end 
of  1883,  followed  by  an  average  minimum  in  the  middle 
of  1889.  The  next  and  last  maximum  was  passed  in 
1893 ;  it  was  not  a  very  high  one,  perhaps  about  70 — 
but  Wolf  died  in  1893,  and  we  have  no  authentic  figures 
later  than  1891. 

A  moment's  inspection  of  the  curve  shows  that  the 
maxima  differ  greatly  in  intensity,  and  that  the  period 
is  not  at  all  fixed  and  certain  like  that  of  an  orbital  mo- 
tion, but  is  subject  to  great  variations.  Thus,  between 
the  maxima  of  1829'9  and  1837*2  we  have  an  interval 
of  only  7'3  years,  while  between  1788  and  1804  it  was 
16'1  years.*  A  portion  of  this  great  variableness  of 
period  may,  perhaps,  be  due  to  the  incompleteness  of 
our  observations,  but  only  a  portion.  It  is  quite  likely 
that  a  fluctuation  of  much  longer  period,  not  far  from 
sixty  years,  is,  to  some  extent,  responsible  for  the  effect 
by  its  superposition  upon  the  principal  (eleven-year) 
oscillation. 

Another  important  fact  is  that  the  interval  from  a 
minimum  to  the  next  following  maximum  is  only  about 
4£  years  on  the  average,  while  from  the  maximum  to 
the  next  following  minimum  the  interval  is  6*6  years. 
The  disturbance  which  produces  the  sun-spots  springs 
up  suddenly,  but  dies  away  gradually. 

Still  another  fact,  as  yet  unexplained,  and  probably 
of  great  theoretical  importance,  has  recently  been 
brought  out  by  Spoerer.  Speaking  broadly,  the  dis- 
turbance which  produces  the  spots  of  a  given  sun-spot 
period  first  manifests  itself  in  two  belts  about  30°  north 

*  Some  astronomers  contend  that  there  ought  to  be  another  maximum 
inserted  about  1795.  Observations  about  this  time  are  few  in  number 
and  not  very  satisfactory. 


PERIODICITY   OF  SUN-SPOTS. 


157 


and  south  of  the  sun's  equator.  These  belts  then  draw 
in  toward  the  equator,  and  the  sun-spot  maximum  occurs 
when  their  latitude  is  about  16°  ;  while  the  disturbance 
gradually  and  finally  dies  out  at  a  latitude  of  8°  or  10°, 
some  twelve  or  fourteen  years  after  its  first  outbreak. 
Two  or  three  years  before  this  disappearance,  however, 
two  new  zones  of  disturbance  show  themselves.  Thus, 
at  the  sun-spot  minimum  there  are  four  well-marked 
spot-belts ;  two  near  the  equator,  due  to  the  expiring 
disturbance,  and  two  in  high  latitudes,  due  to  the  newly 
beginning  outbreak ;  and  it  appears  that  the  true  sun- 
spot  cycle  is  from  twelve  to  fourteen  years  long,  each  be- 
ginning in  high  latitudes  before  the  preceding  one  has 
expired  near  the  equator. 

Fig.  40  illustrates  this,  embodying  Spoerer's  results 
from  1855  until  1880.     The  dotted  curves  show  Wolfs 

FIG.  40. 


18 

55                  18 

60                 18 

65                  18 

70                   18 

75                  18 

80 

L. 

3°; 

26 
22° 
18° 
14° 
10° 

f» 
/ 

\ 

\ 

W. 

100 

50 
0 

\ 

\ 

< 

/• 

\ 

\ 

X 

\ 

Nt 

' 

1 
"^ 

'--- 

\ 

I 

^ 

/ 

"^ 

s, 

"•^ 

-V 

\ 

'v 

\ 

\ 

1 

"X 

\ 

^ 

^ 

\ 

\ 

-- 

/ 

SPOERER'S  CURVES  OF  SUN-SPOT  LATITUDE. 

sun-spot  curve  for  that  period,  the  vertical  column  at 
the  right  of  the  figure,  marked  W  at  the  top,  giving 
"Wolf's  "  relative  numbers"  The  two  continuous  curves, 
on  the  other  hand,  give  the  solar  latitudes  of  the  two 
series  of  spots  that  invaded  the  sun's  surface  in  those 
years.  The  scale  of  latitudes  is  on  the  left  hand.  The 
first  series  began  in  1856  and  ended  in  1868  ;  the  second 
broke  out  in  1866  and  lasted  until  1880.  During  these 


158  THE   SUN. 

years  it  happened  that  there  was  very  little  difference 
between  the  northern  and  southern  hemispheres  of 
the  sun. 

EXPLANATIONS    OF    SUN-SPOT    PERIODICITY. 

There  is  no  question  of  solar  physics  more  interest- 
ing or  important  than  that  which  concerns  the  cause  of 
this  periodicity,  but  a  satisfactory  solution  remains  to 
be  found.  It  has  been  supposed  by  astronomers  of 
very  great  authority  that  the  influence  of  the  planets 
in  some  way  produces  it.  Jupiter,  Yenus,  and  Mer- 
cury have  been  especially  suspected  of  complicity  in 
the  matter,  the  first  on  account  of  his  enormous  mass, 
the  others  on  account  of  their  proximity.  De  La  Rue 
and  Stewart  deduced  from  their  photographic  observa- 
tions of  sun-spots,  between  1862  and  1866,  a  series  of 
numbers,  strongly  tending  to  prove  that,  when  two  of 
the  powerful  planets  are  nearly  in  line  as  seen  from 
the  sun,  then  the  spotted  area  is  much  increased.  They 
have  investigated  especially  the  combined  effect  of  Mer- 
cury and  Venus,  Jupiter  and  Venus,  and  Jupiter  and 
Mercury,  as  also  the  effect  of  Mercury's  approach  to, 
or  recession  from,  the  sun.  In  all  four  cases  therfe 
seems  to  be  a  somewhat  regular  progression  of  num- 
bers, though  much  less  decided  in  the  third  and  fourth 
than  in  the  first  and  second.  The  irregular  variations 
of  the  numbers  are,  however,  so  large,  and  the  duration 
of  the  observations  so  short,  that  it  is  hardly  safe  to 
build  heavily  upon  the  observed  coincidences,  since  they 
may  be  merely  accidental.  In  fact,  so  far  as  we  can 
learn,  the  observations  since  1866  furnish  no  confirma- 
tion of  this  theory. 

An  attempt  to  connect  the  eleven-year  period  with 
that  of  the  planet  Jupiter  also  breaks  down.  While, 


PERIODICITY   OF  SUN-SPOTS.  159 

for  a  certain  portion  of  time,  there  is  a  pretty  good 
agreement  between  the  sun-spot  curve  and  that  which 
represents  the  varying  distance  of  Jupiter  from  the  sun, 
there  is  complete  discordance  elsewhere.  About  1870 
the  maximum  spottedness  occurred  when  the  planet  was 
nearest  the  sun,  but  at  the  beginning  of  the  century  the 
reverse  was  the  case.  Loomis  suggested  that  the  con- 
junctions and  oppositions  of  Jupiter  and  Saturn  may 
be  at  the  bottom  of  the  matter.  These  occur  at  inter- 
vals of  9*93  years,  from  a  conjunction  to  an  opposition, 
or  vice  versa.  But,  when  we  come  to  test  the  matter, 
we  find  that,  in  some  cases,  sun-spot  minima  have  coin- 
cided with  this  alignment  of  the  two  planets ;  in  other 
cases,  maxima. 

It  is,  indeed,  very  difficult  to  conceive  in  what  man- 
ner the  planets,  so  small  and  so  remote,  can  possibly 
produce  such  profound*  and  extensive  disturbances  on 
the  sun.  It  is  hardly  possible  that  their  gravitation 
can  be  the  agent,  since  the  tide-raising  power  of  ^enus 
upon  the  solar  surface  would  be  only  about  T^  of  that 
which  the  sun  exerts  upon  the  earth ;  and  in  the  case  of 
Mercury  and  Jupiter  the  effect  would  be  still  less,  or 
about  10100  of  the  sun's  influence  on  the  earth.  Making 
all  allowances  for  the  rarity  of  the  materials  which  com- 
pose the  photosphere,  it  is  quite  evident  that  no  planet- 
lifted  tides  can  directly  account  for  the  phenomena. 
If  the  sun-spots  are  due  in  any  way  to  planetary  action, 
this  action  must  be  an  occasion  rather  than  a  cause.  A 
minute  disturbance  may,  so  to  speak,  "  pull  the  trigger  " 
and  bring  on  an  explosion.  The  touch  of  a  child's 
finger  fired  the  Flood  Rock  mine. 

Several  astronomers,  among  others  Professor  B. 
Peirce,  seem  to  have  adopted  an  idea  before  alluded 
to — first  suggested,  we  believe,  by  Sir  John  Herschel — 


160  THE  SUN. 

that  the  spots  are  caused  by  meteors  falling  upon  the 
sun.  According  to  this  view,  the-  periodicity  of  the 
spots  could  be  simply  accounted  for  by  supposing  the 
meteors  to  move  in  a  very  elongated  orbit,  with  a  pe- 
riod of  ll'l  years,  adding  the  additional  hypothesis 
that  at  one  part  of  the  orbit  they  form  a  flock  of  great 
density,  while  elsewhere  they  are  sparsely  distributed. 
This  meteoric  orbit  would  have  to  lie  nearly  in  the 
plane  of  the  sun's  equator,  and  have  its  aphelion  near 
the  orbit  of  Saturn.  Of  course  there  is  no  necessity  to 
limit  our  hypothesis  to  a  single  meteor-stream.  What 
we  know  of  meteor-showers  encountered  by  the  earth, 
makes  it  likely  that  there  may  be  several,  of  different 
periods ;  and  thus  we  may  account  for  some  of  the  ob- 
served irregularities  of  the  sun-spot  period.  The  hy- 
pothesis has  many  excellent  points,  and  we  shall  have 
occasion  to  recur  to  it  again.  At  the  same  time,  it  may 
be  said  here  that  it  seems  very  difficult  to  make  it  ex- 
plain the  enormous  dimensions  and  persistence  of  many 
sun-spot  groups,  and  the  distribution  of  the  spots  on 
the  sun's  surface  in  two  parallel  zones,  with  a  minimum 
at  the  equator.  The  irregularity  in  the  epochs  of  max- 
ima and  minima  is  also  much  greater  than  would  have 
been  expected. 

On  the  whole,  it  seems  rather  more  probable  that 
the  periodicity  is  in  the  sun  itself,  depending  upon  no 
external  causes,  but  upon  the  constitution  of  the  photo- 
sphere and  the  rate  at  which  the  sun  is  losing  heat. 
Perhaps  we  may  compare  small  things  with  great  by 
referring  to  the  periodic  explosions  of  the  Icelandic 
geysers,  or  the  "  bumping "  of  ether  and  many  other 
liquids  in  a  chemist's  test-tube.  Looking  at  it  in  this 
light,  we  should  imagine  the  course  of  events  to  consist 
of  a  gathering  of  deep-lying  forces  during  a  season  of 


PERIODICITY   OF  SUN-SPOTS.  161 

external  quiescence,  followed  by  an  outburst,  which 
relieves  the  internal  fury ;  the  rest  and  the  paroxysms 
recurring,  at  somewhat  regular  intervals,  simply  because 
the  forces,  materials,  and  conditions  involved,  change 
only  slowly  with  the  lapse  of  time. 

If  such  be  really  the  case,  it  is  clear,  of  course,  that 
this  periodicity  is  never  likely  to  be  very  regular,  and 
will  not  long  keep  step  with  any  planetary  march. 
Time  of  itself,  therefore,  will  by-and-by  solve  the  prob- 
lem for  us,  or  at  least  will  refute  any  false  hypothesis 
resting  upon  the  recurrence  of  planetary  positions. 

TERRESTRIAL    INFLUENCE    OF    SUN-SPOTS. 

Even  more  important  than  the  problem  of  the  cause 
of  sun-spot  periodicity,  is  the  question  whether  this  pe- 
riodicity produces  any  notable  effects  upon  the  earth, 
and,  if  so,  what  ?  In  regard  to  this  question  the  astro- 
nomical world  is  divided  into  two  almost  hostile  camps, 
so  decided  is  the  difference  of  opinion,  and  so  sharp  the 
discussion.  One  party  holds  that  the  state  of  the  sun's 
surface  is  a  determining  factor  in  our  terrestrial  meteor- 
ology, making  itself  felt  in  our  temperature,  barometric 
pressure,  rainfall,  cyclones,  crops,  and  even  our  financial 
condition,  and  that,  therefore,  the  most  careful  watch 
should  be  kept  upon  the  sun  for  economic  as  well  as 
scientific  reasons. 

The  other  party  contends  that  there  is,  and  can  be, 
no  sensible  influence  upon  the  earth  produced  by  such 
slight  variations  in  the  solar  light  and  heat,  though,  of 
course,  they  all  admit  the  connection  between  sun-spots 
and  the  condition  of  the  earth's  magnetic  elements.  It 
seems  pretty  clear  that  we  are  not  in  a  position  yet  to 
decide  the  question  either  way;  it  will  take  a  much 
longer  period  of  observation,  and  observations  con- 
12 


162  THE  SUN- 

ducted  with  special  reference  to  the  subject  of  inquiry, 
to  settle  it.  At  any  rate,  from  the  data  now  in  our  pos- 
session, men  of  great  ability  and  laborious  industry 
draw  opposite  conclusions. 

It  certainly  is  not  so  plain  that  the  sun-spots  have 
not  the  influence  which  their  worshipers,  I  had  almost 
called  them,  claim  for  them,  as  to  absolve  us  from 
the  duty  of  investigating  the  matter  in  the  most  thor- 
ough manner.  On  the  other  hand,  it  is  also  by  no 
means  certain  that  we  shall  find  the  labor  of  investiga- 
tion fruitful  in  precisely  the  manner  and  degree  desired. 
Those  who  search  for  truth  with  honest  endeavor  may, 
nevertheless,  be  sure  of  their  reward  in  some  way. 

I  have  said  that  there  is  no  doubt  as  to  the  con- 
nection between  the  sun-spots  and  terrestrial  magnetism. 

In  1850,  Lamont,  of  Munich,  called  attention  to  the 
fact  that  the  average  daily  excursions  of  the  magnetic 
needle  have  a  period  which,  from  the  few  decades  of 
observation  at  his  command,  he  fixed  at  ten  and  one 
third  years. 

Perhaps  a  word  of  explanation  is  needed  here. 
Every  one  knows  that  the  compass-needle  does  not 
point  exactly  north,  and  its  divergence  from  the  true 
meridian  is  different  in  different  places.  On  the  At- 
lantic coast  of  the  United  States,  for  instance,  the  north 
pole  of  the  magnet  points  west  of  north,  and  on  the 
Pacific  coast  east  of  north.  "What  is  more :  at  any  par- 
ticular place  the  direction  of  the  needle  is  continually 
changing,  these  changes  being  like  the  changes  in  the 
temperature  of  the  air,  in  part  regular  and  predictable, 
and  partly  lawless,  so  far  as  we  can  see. 

One  of  the  most  noticeable  of  the  regular  magnetic 
changes  is  the  so-called  diurnal  oscillation  ;  during  the 
early  part  of  the  day,  between  sunrise  and  one  or  two 


MAGNETISM  AND  SUN-SPOTS.  163 

o'clock  p.  M.,  the  north  pole  of  the  needle  moves  toward 
the  west  in  these  latitudes,  returning  to  its  mean  position 
about  10  P.  M.,  and  remaining  nearly  stationary  during 
the  night.  The  extent  of  this  oscillation  in  the  United 
States  is  about  15'  of  arc  in  summer,  and  not  quite  half 
as  much  in  winter ;  but  it  differs  very  much  in  different 
localities  and  at  different  times,  and  also — and  this  is 
Lamont's  discovery — the  average  extent  of  this  diurnal 
oscillation  at  any  given  observatory  increases  and  de- 
creases pretty  regularly  during  a  period  of  lOj-  years, 
according  to  his  calculations.  As  soon  as  Schwabe  an- 
nounced his  discovery  of  the  periodicity  of  the  solar 
spots,  Sabine  in  England,  Gautier  in  France,  and  Wolf 
in  Switzerland,  at  once  and  independently  perceived  the 
coincidence  between  the  spot-maxima  and  those  of  the 
magnetic  oscillation.  Faye  at  one  time  attempted  to 
impugn  this  conclusion.  In  order  to  make  his  point,  he 
insisted  that  the  magnetic  maximum  is  shown  by  Cas- 
sini's  observations  to  have  occurred  early  in  1787,  and, 
dividing  the  interval  between  this  and  the  last  magnetic 
maximum,  near  the  close  of  1870,  by  8,  the  number  of 
intervening  periods,  he  gets  10*45  years  for  the  mean 
magnetic  period,  instead  of  11*11.  The  reply  is,  that 
the  observations  both  of  the  sun-spots  and  of  the  mag- 
netic elements  near  the  close  of  the  eighteenth  century 
are  so  meager  and  unsatisfactory  that  the  evidence  as 
to  the  precise  time  of  maxima  and  minima  is  very  in- 
complete. In  1885,  however,  Faye  yielded  to  the  con- 
stantly accumulating  weight  of  evidence,  and  gave  in 
his  adhesion  to  the  received  conclusion,  which  is  now 
practically  undisputed. 

The  convincing  evidence  as  to  the  reality  of  the  as- 
serted connection  lies  in  the  closeness  with  which,  ever 
since  we  have  been  in  possession  of  continuous  and  sat- 


164  THE  SUN. 

isf  actory  observations,  the  magnetic  curve  copies  that  of 
the  sun-spots.  In  Fig.  39  the  dotted  curves  represent 
the  mean  amount  of  magnetic  oscillation  as  deduced  by 
Wolf  from  various  series  of  observations.  From  1820 
to  1895  the  record  is  almost  continuous,  and  the  coinci- 
dence of  the  curves  is  such  as  to  make  it  impossible  to 
doubt  the  connection.* 

The  argument  is  much  strengthened  by  an  examina- 
tion of  records  of  the  aurora  borealis.  Occasionally  so- 
called  "  magnetic  storms  "  occur,  during  which  the  com- 
pass-needle is  sometimes  almost  wild  with  excitement, 
oscillating  5°  or  even  10°  within  an  hour  or  two.  These 
"  storms  "  are  generally  accompanied  by  an  aurora,  and 
an  aurora  is  always  accompanied  by  magnetic  disturb- 
ance. 

Now,  when  we  come  to  collate  aurora  observations 
with  those  of  sun-spots,  as  Loomis  has  done  with  great 
care  and  thoroughness,  we  find  an  almost  perfect  paral- 
lelism between  the  curves  of  auroral  and  sun-spot  fre- 
quency. 

We  find  also,  as  Shearman,  of  Toronto,  and  Dr. 
Yeeder,  of  Lyons,  N.  Y.,  have  pointed  out,  that  auroras 
often  run  in  series,  so  to  speak,  following  each  other  for 
several  months  at  nearly  regular  intervals  of  27*275 
days,  which  is  very  closely  the  period  of  the  sun's  ap- 
parent equatorial  (synodic)  rotation ;  this  of  course 
makes  it  more  or  less  probable  that  their  appearance  is 
connected  somehow  with  the  way  in  which  certain  por- 
tions of  the  sun's  surface  present  themselves  to  the 
earth.  Dr.  Yeeder's  idea  is  that  disturbed  regions  upon 

*  A  discussion,  by  Balfour  Stewart,  of  the  observations  at  Kew,  be- 
tween 1856  and  1867,  brings  out  the  correspondence  very  beautifully, 
and  seems  to  show  that  the  magnetic  changes  lag  behind  the  sun-spots 
about  five  months. 


MAGNETISM   AND   SUN-SPOTS.  165 

the  sun  are  specially  influential  upon  the  earth's  mag- 
netism at  the  moment  when  they  are  near  the  eastern 
edge  of  the  sun,  and  just  coming  in  sight  to  us  on  the 
earth.  There  is  no  obvious  reason,  however,  why  a 
disturbance  on  the  sun  should  thus  propagate  itself  more 
vigorously  in  a  direction  tangential  to  the  sun's  surface 
and  in  the  plane  of  the  sun's  equator  than  in  any 
other  direction  ;  and  while  Dr.  Veeder  is  certainly  able 
to  marshal  a  great  number  of  coincidences  in  support 
of  his  opinion,  there  are  also  numerous  cases  where  the 
region  of  solar  disturbance  was  near  the  middle  of  the 
sun's  disk,  as,  for  instance,  the  great  magnetic  storms  and 
auroras  of  February  13,  1892,  and  November  IT,  1882. 
In  this  connection  it  seems  worth  while  to  quote  from 
an  article  by  Mr.  Maunder,  of  the  Greenwich  Observa- 
tory, with  respect  to  these  spots,  and  two  other  groups 
of  almost  equal  magnitude  which  appeared  together  in 
April,  1882.  He  writes  : 

44  In  a  period  of  nearly  nineteen  years,  therefore  "  (from  1873 
to  1892),  "  we  have  three  magnetic  storms  which  stand  out  pre- 
eminently above  all  others  during  that  interval.  In  that  same 
period  we  have  three  great  sun-spot  displays — counting  the  two 
groups  of  April,  1882,  together — which  stand  out  with  equal  dis- 
tinctness far  above  all  other  similar  displays.  And  we  find  that 
the  three  magnetic  storms  were  simultaneous  with  the  greatest  de- 
velopment of  the  spots.  Is  there  any  escape  from  the  conclusion 
that  the  two  have  a  real  and  binding  connection?  It  may  be  di- 
rect ;  it  may  be  indirect  and  secondary  only ;  but  it  must  be  real 
and  effective."— "Knowledge,"  May,  1892. 

It  is  not  easy  to  frame  any  satisfactory  theory  to  ac- 
count for  this  connection  between  solar  disturbances  and 
terrestrial  magnetism.  It  can  hardly  be  in  the  way  of 
temperature,  for  the  influence  of  sun-spots  in  this  re- 
spect is  so  slight  that  it  is  still  an  open  question  whether 
we  do  or  do  not  get  from  the  sun  more  than  the  average 


166  THE  SUN. 

amount  of  heat  during  a  sun-spot  maximum.  Probably 
it  is  more  immediate  and  direct ;  perhaps  in  some  way 
kindred  with  the  action  which  drives  off  the  material  of 
a  comet's  tail,  and  proves  that  other  forces  besides  grav- 
itation are  operative  in  interplanetary  space.  Or,  not 
at  all  impossibly,  as  Mr.  Maunder  suggests,  it  may  be 
indirect ;  an  action  of  some  cosmic  cause  upon  sun  and 
earth  together. 

There  are  a  number  of  observed  instances  which, 
though  not  sufficient  to  demonstrate  the  fact,  still  ren- 
der it  very  probable  that  every  intense  disturbance  of 
the  solar  surface  is  propagated  to  our  terrestrial  mag- 
netism with  the  speed  of  light.  An  instance  fell  under 
the  writer's  notice  in  the  course  of  a  series  of  spectro- 
scopic  observations  at  Sherman.  On  August  3,  1872, 
the  chromosphere  in  the  neighborhood  of  a  sun-spot, 
which  was  just  coming  into  view  around  the  edge  of  the 
sun,  was  greatly  disturbed  on  several  occasions  during 
the  forenoon.  Jets  of  luminous  matter  of  intense  bril- 
liance were  projected,  and  the  dark  lines  of  the  spectrum 
were  reversed  by  hundreds  for  a  few  minutes  at  a  time. 
There  were  three  especially  notable  paroxysms  at  8.45, 
10.30,  and  11.50  A.  M.  local  time.  At  dinner  the  pho- 
tographer of  the  party,  who  was  determining  the  mag- 
netic constants  of  our  station,  told  me,  without  knowing 
anything  about  my  observations,  that  he  had  been  obliged 
to  give  up  work,  his  magnet  having  swung  clear  off  the 
scale.  Two  days  later  the  spot  had  come  around  the 
edge  of  the  limb.  On  the  morning  of  August  5th  I 
began  observations  at  6.40,  and  for  about  an  hour  wit- 
nessed some  of  the  most  remarkable  phenomena  I  have 
ever  seen.  The  hydrogen  lines,  with  many  others,  were 
brilliantly  reversed  in  the  spectrum  of  the  nucleus,  and 
at  one  point  in  the  penumbra  the  C  line  sent  out  what 


MAGNETISM  AND   SUN-SPOTS. 


167 


looked  like  a  blowpipe-jet,  projecting  toward  the  up- 
per end  of  the  spectrum,  and  indicating  a  motion  along 
the  line  of  sight  of  about  one  hundred  and  twenty  miles 
per  second.  This  motion  would  die  out  and  be  renewed 
again  at  intervals  of  a  minute  or  two.  The  figure  gives 
an  idea  of  the  appearance  of  the  spectrum.  The  dis- 
turbance ceased  before  eight  o'clock  and  was  not  re- 
newed that  forenoon.  On  writing  to  England,  I  re- 

Fio.  41. 


ceived  from  Greenwich  and  Stony  hurst,  through  the 
kindness  of  Sir  G.  B.  Airy  and  Rev.  S.  J.  Perry,  copies 
of  the  photographic  magnetic  records  for  those  two 
days.  Fig.  42  is  reduced  from  the  Greenwich  curve. 
That  obtained  at  Stonyhurst  is  essentially  the  same.  It 
will  be  seen  that  on  August  3d,  which  was  a  day  of  gen- 
eral magnetic  disturbance,  the  three  paroxysms  I  noticed 
at  Sherman  were  accompanied  by  peculiar  twitches  of  the 
magnets  in  England.  Again,  August  5th  was  a  quiet 
day,  magnetically  speaking,  but  just  during  that  hour 
when  the  sun-spot  was  active,  the  magnet  shivered  and 


168 


THE   SUN. 


trembled.  So  far  as  appears,  too,  the  magnetic  action 
of  the  sun  was  instantaneous.  After  making  allowance 
for  longitude,  the  magnetic  disturbance  in  England  ap- 
pears strictly  simultaneous,  so  far  as  can  be  judged, 


FIG.  42. 


MAGNETIC  CUEVKS  AT  GREENWICH  (August  3  and  5,  1872). 

with  the  spectroscopic  disturbance  seen  on  the  Rocky 
Mountains,  and  the  difference  can  not  have  been  more 
than  about  ten  minutes.  But  the  time  at  Sherman 
was  not  noted  with  any  great  precision. 

Of  course,  as  has  been  said,  no  two  or  three  coinci- 
dences such  as  have  been  adduced  are  sufficient  to  es- 
tablish the  doctrine  of  the  sun's  immediate  magnetic 
action  upon  the  earth,  but  they  make  it  so  far  probable 
as  to  warrant  a  careful  investigation  of  the  matter — an 
investigation,  however,  which  is  not  easy,  since  it  im- 
plies a  practically  continuous  watch  of  the  solar  surface. 


MAGNETISM  AND  SUN-SPOTS.  169 

It  may  be  added,  too,  that  many  striking  disturb- 
ances which  have  been  observed  upon  the  sun,  in  the 
ascent  of  lofty  prominences,  received  no  magnetic  re- 
sponse from  the  earth ;  and  there  have  also  been  great 
auroras  with  no  obvious  solar  correlative.  Indeed, 
there  is  every  reason  to  suppose  that  a  large  proportion 
of  all  the  magnetic  disturbances  at  any  given  observa- 
tory are  purely  local,  having  nothing  whatever  to  do 
with  the  sun.  Some  also  which  are  not  local  have 
been  traced  to  the  action  of  the  moon,  and  it  is  not  at 
all  improbable  that  others  yet  are  due  to  causes  oper- 
ating in  interplanetary  space. 

Solar  disturbances  are  not  the  cause  of  our  magnetic 
storms,  but  only  one  cause  of  some  of  them ;  and  very 
likely  a  cause  only  in  the  sense  that  the  pulling  of  a 
trigger  "  causes  "  the  flight  of  a  rifle-ball :  there  need  be 
no  proportionality  between  such  a  cause  and  its  effect. 

It  would  be  unfair  to  our  readers  to  pass  without 
notice  the  remarks  of  Lord  Kelvin  in  a  recent  presiden- 
tial address  to  the  Royal  Society  (November,  1892),  ex- 
pressing his  dissent  from  the  accepted  view  of  the 
relation  we  have  been  discussing.  Taking  the  magnetic 
storm  of  June  25, 1885,  as  an  example,  he  computes  that 

"  In  this  eight  hours  of  a  not  very  severe  magnetic  storm  as  much 
work  must  have  been  done  by  the  sun  in  sending  magnetic  waves 
out  in  all  directions  through  space  as  he  actually  does  in  four 
months  of  his  regular  heat  and  lipht.  This  result,"  he  adds,  "it 
seems  to  me,  is  absolutely  conclusive  against  the  supposition  that 
terrestrial  magnetic  storms  are  due  to  magnetic  action  of  the 
sun,  or  to  any  kind  of  action  taking  place  within  the  sun,  or  in 
connection  with  hurricanes  in  his  atmosphere,  or  anywhere  near 
the  sun  outside.  It  seems  as  if  we  may  also  be  forced  to  con- 
clude that  the  supposed  connection  between  magnetic  storms  and 
sun-spots  is  unreal,  and  that  the  seeming  agreement  between  the 
periods  has  been  a  mere  coincidence." 


170  THE  SUN. 

And  yet,  with  all  deference  to  so  high  an  authority, 
and  without  questioning  the  accuracy  of  his  calculations, 
they  seem  really  to  be  no  more  conclusive  than  a  com- 
putation to  show  that  the  work  done  by  an  explosion 
vastly  exceeded  the  power  of  the  person  who  pressed 
the  firing  button.  The  nature  of  the  mechanism  by 
which  the  connection  is  established  may,  and  still 
does,  remain  uncertain,  but  the  statistics  leave  no  doubt 
as  to  the  reality  of  the  connection  itself.  It  is  not,  per- 
haps, outside  the  limits  of  possibility,  as  before  hinted, 
that  both  the  solar  and  terrestrial  disturbance  have  a 
common  origin  in  some  invasion  of  power  or  matter 
from  outer  space  —  that  the  solar  tumult  is  the  brother 
and  not  the  father  of  our  own  aurora. 

As  to  the  effect  of  sun-spots  upon  terrestrial  temper- 
ature, no  conclusion  seems  possible  at  present.  The 
spots  themselves,  as  Henry,  Secchi,  Langley,  and  others 
have  shown,  certainly  radiate  to  us  less  heat  than  the 
general  surface  of  the  sun.  According  to  the  elaborate 
determinations  of  Langley,  the  umbra  of  a  spot  emits 
about  fifty-four*  per  cent,  and  the  penumbra  about 
eighty  per  cent,  as  much  heat  as  a  corresponding  area 
of  the  photosphere.  The  direct  effect  of  sun-spots  is, 
therefore,  to  make  the  earth  cooler.  As  the  total  area 
covered  by  spots,  even  at  the  time  of  maximum,  never 
exceeds  -g-J-g-  of  the  whole  surface  of  the  sun,  it  follows 


*  The  most  recent  observations,  those  made  at  Daramona,  in  Ireland, 
by  W.  E.  Wilson,  in  1893,  with  a  "  radio-micrometer  "  and  other  apparatus 
of  the  highest  order,  give  about  forty-six  per  cent,  for  this  ratio.  All  ob« 
servers  find  that  it  increases  near  the  limb  of  the  sun,  and  both  Langley 
and  Frost  have  encountered  cases  where  the  umbra  of  a  spot  was  appar- 
ently warmer  than  the  surrounding  photosphere  :  a  fact  which,  if  not  the 
result  of  some  error  of  observation,  is  difficult  to  explain  on  the  theory 
that  spots  are  cavities,  though  a  necessary  consequence  if  they,  like  the 
f  aculas,  are  masses  floating  at  some  elevation  above  the  photosphere. 


MAGNETISM  AND  SUN-SPOTS. 

that  directly  they  may  diminish  our  heat-supply  by 
about  yjnnr  °^  *he  whole.  Whether  this  effect  would 
be  sensible  or  not,  is  a  question  not  easily  answered. 

But,  while  the  direct  effect  would  be  of  this  nature, 
it  is  quite  probable  that  it  is  at  least  fully  compensated 
by  another  of  the  opposite  character.  We  get  our  light 
and  heat  from  the  photosphere  which  is  covered  by  an 
atmosphere  of  gases,  and  in  this  atmosphere  a  consider- 
able absorption  occurs.  Now,  if  the  level  of  the  photo- 
spheric  surface  be  disturbed,  so  that  it  is  covered  with 
waves  and  elevations  of  any  considerable  height,  as 
compared  with  the  thickness  of  the  overlying  atmos- 
phere, then,  as  Langley  has  shown,  the  radiation  will 
at  once  be  increased ;  since,  while  the  absorption  is  in- 
creased by  a  certain  percentage  for  those  portions  of 
the  photosphere  which  are  depressed  below  their  ordi- 
nary level,  it  is  much  more  decreased  for  those  that  are 
raised. 

The  reason  of  this  is  that,  when  a  luminous  object 
is  immersed  in  an  absorbing  medium  it  loses  much 
more  light  for  the  first  foot  of  submergence  than  for 
the  second,  and  more  for  the  second  than  for  the  third ; 
so  that  when  it  has  reached  a  considerable  depth  it  re- 
quires an  additional  submergence  of  many  feet  to  di- 
minish its  radiation  as  much  as  the  first  foot  did.  If, 
therefore,  sun-spots  are  accompanied  by  considerable 
vertical  disturbance  of  the  photosphere,  as  is  almost 
certain,  we  must  have  as  a  result  an  increased  radia- 
tion on  account  of  the  disturbance,  offsetting,  more  or 
less  entirely,  the  opposite  effect  which  is  at  first  view 
most  obvious. 

Then,  again,  it  is  altogether  probable  that  spots  are 
either  due  to,  or  accompanied  by,  an  eruptive  action — 
the  internal,  and  hotter,  gases  bursting  through  the  pho- 


172  THE   SUN. 

tosphere  with  unusual  abundance  during  seasons  of 
spot-maximum.  This  must  necessarily  tend  to  increase 
the  emission  of  heat  from  the  sun,  and  possibly  by  a 
considerable  amount.  But,  on  the  other  hand,  any 
considerable  increase  in  the  thickness  of  the  chromo- 
sphere, such  as  might  result  from  abundant  and  long- 
continued  eruption,  would  work  in  thft  opposite  direc- 
tion. 

It  is  impossible,  therefore,  to  predict,  a  priori,  which 
effect  will  predominate,  or  to  say  whether  the  mean 
temperature  of  the  earth  ought  to  be  raised  or  lowered 
during  a  sun-spot  maximum  ;  and  thus  far  no  compari- 
son of  observations  has  settled  the  matter  to  general 
satisfaction.  At  least,  no  longer  ago  than  1878,  Balfour 
Stewart,  who  ought  to  know  if  any  one,  writes,  "  It  is 
nearly,  if  not  absolutely,  impossible,  from  the  observa- 
tions already  made,  to  tell  whether  the  sun  be  hotter  or 
colder,  as  a  whole,  when  there  are  most  spots  on  his 
surface." 

On  the  one  hand,  Jelinek,  from  all  temperature 
observations  available  in  Germany  up  to  1870,  found 
the  influence  of  sun-spots  entirely  inappreciable,  though 
from  the  same  observations  he  did  deduce  minute  effects 
produced  by  the  changes  in  the  distance  and  phase  of 
the  moon.  On  the  other  hand,  Mr.  Stone,  while  astrono- 
mer royal  at  the  Cape  of  Good  Hope,  and  Dr.  Gould,  in 
South  America,  consider  that  the  observations  taken  at 
their  stations  show  a  distinct  though  slight  diminution 
of  temperature  at  the  time  of  a  sun-spot  maximum : 
according  to  Dr.  Gould  the  difference  at  Buenos  Ayres 
between  maximum  and  minimum  amounts  to  about 
1J°  Fahr.  He  also  considers  that  the  meteorological 
records  of  the  Argentine  Republic  between  1875  and 
1885  show  a  distinct  connection  between  the  sun-spots 


METEOROLOGY  AND  SUN-SPOTS.  173 

and  the  force  and  direction  of  the  winds  at  the  various 
stations.  At  the  Cape  of  Good  Hope,  Mr.  Stone  finds 
the  difference  to  be  about  three  fourths  of  a  degree 
from  thirty  years'  observations — at  least,  if  we  rightly 
interpret  his  curve  of  temperatures,  for  it  is  not  quite 
clear  what  unit  of  temperature  is  used  in  constructing 
his  diagram. 

At  Edinburgh,  Piazzi  Smyth  finds  in  the  records 
of  the  rock  thermometers  a  marked  eleven-year  perio- 
dicity, of  which  the  range  amounts  to  about  a  degree 
(Fahr.),  and  the  maxima,  instead  of  coinciding  with  the 
sun-spot  minima,  come  about  two  years  behind  them. 

On  the  whole,  perhaps,,. as  things  now  stand,  it  would 
be  fair  to  say  that  there  is  a  small  balance  of  probability 
in  favor  of  the  statement  that  years  of  sun-spot  maxi- 
mum are  a  degree  or  so  cooler  than  those  of  spot-mini- 
mum ;  but  the  balance  is  very  slight  indeed,  and  the 
next  investigation  of  somebody  else  may  carry  it  to  the 
other  side. 

As  regards  the  influence  of  sun-spots  upon  storms 
and  rainfall,  the  evidence,  if  not  entirely  conclusive,  as 
it  is  considered  by  Mr.  Lockyer  and  some  other  high 
authorities,  is  at  least  considerably  stronger.  In  18Y2 
Mr.  Meldrum,  director  of  the  observatory  at  the  Mau- 
ritius, published  a  comparison  between  the  number  of 
cyclones  observed  in  the  Indian  Ocean  and  the  state  of 
the  sun,  and  pointed  out  that  the  number  of  cyclones 
was  greatest  at  the  time  of  a  sun-spot  maximum.  We 
quote  his  words  ("  Nature,"  vol.  vi,  p.  358) :  "  Taking 
the  maxima  and  minima  epochs  of  the  sun-spot  period, 
and  one  year  on  each  side  of  them,  and  comparing  the 
number  of  cyclones  in  these  three-year  periods,  we  get 
the  following  results : 


174 


THE   SUN. 


YBABS. 

No.  of  cyclones 
in  each  year. 

Total  No.  of 
cyclones. 

11847  

4  ) 

1848 

et 

15 

1849  

M 

1 

11855  

4  ) 

Minima    .  .  •> 

1856                   .              

it 

8 

\ 

1857.   

81 

1859 

5  ) 

Maxima     . 

I860  . 

U 

21 

1861  

si 

1866    

5  ) 

Minima.  ,  .  . 

1867  

8> 

9 

1868 

2J 

1870  

3  ) 

Maxima  .  .  . 

1871  

*•* 

14" 

1872  

7j 

Subsequently  Mr.  Meldrura  made  more  extensive 
comparisons,  including  not  only  cyclones  proper,  but 
other  great  storms,  and  brings  out  essentially  the  same 
results.  At  the  same  time  it  is  to  be  noted  that  the 
yearly  numbers  vary  enormously,  and,  on  referring  to 
his  second  paper  ("  Nature,"  vol.  viii,  p.  495),  it  will  be 
found  that  the  number  for  the  sun-spot  maximum, 
lS47-'49,  is  only  twenty-three,  while  that  for  the  mini- 
mum, 1866-'68,  is  twenty-one.  (Mr.  Meldrum  coaxes 
the  first  sun-spot  maximum  a  little  by  using  the  years 
1848-'50  in  his  comparison ;  rather  unwarrantably,  it 
would  seem,  since  the  epoch  of  spot  maximum  was 
1848*1 :  by  using  those  years,  he  gets  twenty-six  instead 
of  twenty-three.) 

The  variations  from  year  to  year  are  so  extreme  that 
it  is  sufficient  to  say  that  the  observations  can  hardly  be 
considered  as  demonstrative  without  much  further  con- 
firmation from  other  sources. 

Mr.  Meldrum  has  attempted  to  supply  this  confirma- 
tion by  tabulating  the  rainfall  at  a  number  of  stations 


METEOROLOGY  AND   SUN-SPOTS.  175 

in  and  near  the  Indian  Ocean,  and  obtains  a  result  con- 
firmatory on  the  whole,  though  there  are  several  discrep- 
ancies. Mr.  Lockyer,  from  observations  of  the  rainfall 
at  the  Cape  of  Good  Hope  and  Madras,  gets  corrobora- 
tive figures. 

Mr.  Meldrum,  in  a  still  later  paper  published  in  the 
"  Monthly  Notices  of  the  Mauritius  Meteorological  So- 
ciety," for  December,  1878,  discusses  at  length  the 
rainfall  of  more  than  fifty  different  stations  in  all  parts 
of  the  earth,  and  also  the  levels  of  many  of  the  princi- 
pal European  rivers.  The  discussion  covers  nearly  all 
the  available  data  from  1824  to  1867.  It  is  only  just 
to  Mr.  Meldrum  to  say  that  the  treatment  seerns  to  be 
sufficiently  thorough,  perfectly  fair,  and  the  result  on 
the  whole  is  in  favor  of  his  opinion  that  there  is  a  real 
connection  between  the  annual  rainfall  and  the  state  of 
the  solar  surface.  He  finds  the  average  rainfall  for  the 
earth  to  be  about  38*5  inches  annually ;  the  range  be- 
tween the  maximum  and  minimum  is  about  four  inches ; 
and  the  rainfall  maximum  occurs  about  a  year  after  the 
sun-spot  maximum,  though  with  a  good  deal  of  varia- 
tion at  different  stations.  In  some  countries,  indeed, 
and  at  some  times  (in  the  United  States,  for  instance, 
between  1834  and  1843),  the  results  conflict  with  the 
theory,  but  the  general  accordance  is  striking,  and  seems 
to  warrant  his  concluding  statement  that  "the  mean 
rainfalls  of  Great  Britain,  the  Continent  of  Europe, 
America,  and  India,  as  represented  by  all  the  returns 
that  have  been  received,  have,  notwithstanding  anom- 
alies, varied  directly  as  Wolf's  sun-spot  numbers  have 
varied,  and  the  epochs  of  maximum  and  minimum  rain 
have  nearly  coincided  with  those  of  the  sun-spots.  The 
rainfalls  at  five  stations  in  the  southern  hemisphere,  for 
shorter  periods,  give  similar  results." 


176  THE 

Mr.  Symons,  from  the  British  rainfall  of  the  past 
one  hundred  and  forty  years,  gets  an  equivocal  result. 
American  stations,  so  far  as  they  have  been  tested,  are 
on  the  whole  rather  in  opposition  to  those  of  the  Indian 
Ocean,  indicating  somewhat  less  rain  than  usual  during 
a  sun-spot  maximum.  But,  as  any  one  can  see  by  con- 
sulting Mr.  Symons' s  paper  in  "Nature,"  vol.  vii,  pp. 
143-145,  in  which  he  has  tabulated  an  immense  number 
of  rainfall  statistics,  the  evidence  is  extremely  conflict- 
ing— altogether  different  in  force  and  character  from 
that  which  demonstrates  the  magnetic  influence  of  solar 
disturbances. 

Still  other  attempts  have  been  made  to  establish  a 
connection  between  sun-spots  and  various  terrestrial 
phenomena.  Thus,  Dr.  T.  Moffat,  in  1874,  published 
results  tending  to  show  that  in  sun-spot  years  the  aver- 
age quantity  of  atmospheric  ozone  is  somewhat  greater 
than  during  a  spot-minimum. 

Another  eminent  physician,  whose  name  escapes  us, 
endeavored,  some  years  ago,  to  show  that  the  visitations 
of  Asiatic  cholera  are  periodical,  and  that  their  period 
depends  upon  that  of  the  sun-spots,  being  just  once  and 
a  half  as  long — about  fifteen  years.  This  periodicity 
may  be  real,  perhaps  •  but,  if  so,  the  fact  that  the  choL 
era  maxima  are  alternately  synchronous  with  the  max- 
ima and  minima  of  the  spots,  would  be  sufficient  to 
disprove  the  idea  of  any  casual  connection  between  the 
phenomena. 

One  of  the  most  interesting  of  the  essays  in  this 
direction,  is  that  of  Professor  Jevons,  who  sought  to 
show  a  relation  between  sun-spots  and  commercial 
crises.  The  idea  is  by  no  means  absurd,  as  some  have 
declared — it  is  a  mere  question  of  fact.  If  sun-spots 
have  really  any  sensible  effect  upon  terrestrial  meteor- 


INFLUENCE  OF  SUN-SPOTS.  177 

ology,  upon  temperature,  storms,  and  rainfall,  they  must 
thus  indirectly  affect  the  crops,  and  so  disturb  financial 
relations ;  in  such  a  delicate  organization  as  that  of  the 
world's  commerce,  it  needs  but  a  feather-weight,  rightly 
applied,  to  alter  the  course  of  trade  and  credit,  and  pro- 
duce a  "  boom  "  (if  we  may  be  forgiven  the  use  of  so 
convenient  a  word)  or  a  crash. 

We  have  not  time  or  space  to  discuss  Mr.  Jevons's 
paper,  but  must  content  ourselves  with  saying  that,  to 
us  at  least,  the  facts  do  not  seem  fairly  to  warrant  his 
conclusion. 

It  can  do  no  harm  to  reiterate  and  emphasize  what 
was  said  a  few  pages  back,  that  the  question  of  sun-spot 
influence  can  not  be  considered  settled;  and  that  the 
only  method  of  deciding  it  is  by  a  continuous  series  of 
careful  observations,  conducted  specially  for  the  pur- 
pose, or  at  least  conducted  with  reference  to  the  con- 
ditions of  the  problem,  since  the  same  observations 
would  also  be  useful  as  data  for  various  other  investi- 
gations. 

"While  it  is  not  at  all  unlikely  that  investigation 
will  result  in  establishing  some  real  influence  of  sun- 
spots  upon  our  terrestrial  meteorology  and  determin- 
ing its  laws,  it  is  practically  certain  that  this  influence 
is  extremely  slight,  and  so  masked  and  veiled  by  other 
influences  more  powerful  that  it  is  extremely  diffi- 
cult to  bring  to  light. 

SUN-SPOT    THEORIES. 

Naturally,  the  remarkable  phenomena  of  the  sun- 
spots  have  invited  speculation  as  to  their  cause. 

As  has  been  mentioned  already,  some  of  the  early 
observers  believed  the  spots  to  be  planetary  bodies  cir- 
culating around  the  sun,  very  near  its  surface.  This 
13 


1Y8  THE  SUN. 

opinion  Galileo  unanswerably  refuted  by  pointing  out 
that  in  that  case  the  spot,  in  its  movement  around  the 
sun,  ought  to  be  visible  less  than  half  the  time.  He, 
on  the  other  hand,  proposed  the  theory  that  they  are 
clouds,  floating  in  the  solar  atmosphere. 

This  view,  in  one  form  or  another,  has  since  been 
held  by  many  astronomers  of  great  authority.  Derharn 
believed  these  clouds  to  be  eruptions  from  solar  volca- 
noes, and  in  our  own  times  Capocci  has  adopted  and 
maintained  the  same  theory.  Peters  seems  to  have  con- 
sidered it  favorably  in  1846,  at  least  so  far  as  the  vol- 
canic part  of  the  hypothesis  is  concerned,  while  Kirch- 
hoff  seems  to  have  assented  to  Galileo's  original  opinion 
unmodified.  If  the  statement  be  interpreted  to  mean 
that  sun-spots  are  masses  of  cloudy  matter,  less  luminous 
than  the  photosphere,  and  floating  in,  not  above,  the 
photosphere,  probably  a  very  large  proportion  of  the 
students  of  solar  physics  would  to-day  agree  to  it.  Gal- 
ileo, however,  believed  the  spot-clouds  to  be  high  above 
the  shining  surface,  which  we  now  know  not  to  be  the 
fact;  for  the  observations  of  Wilson,  in  1769,  men- 
tioned a  few  pages  back,  and  the  whole  body  of  obser- 
vations since  then,  have  made  it  almost  certain  that  the 
umbra  of  a  sun-spot  lies  several  hundred  miles  below 
the  level  of  the  photosphere.* 

Lalande,  however,  was  not  disposed  to  accept  Wil- 
son's doctrine,  and  maintained  that  the  sun-spots  are 
the  tops  of  solar  mountains  projecting  above  the  lumi- 
nous surface — islands  in  the  ocean  of  fire.  In  this  hy- 
pothesis the  penumbra  is  accounted  for  by  the  shelving 
sides  of  the  mountains  seen  through  the  semi-trans- 
parent flame.  It  will  be  noticed  that  the  theories 

*  But  we  must  not  overlook  Mr.  Hewlett's  conclusions  (p.  129),  nor 
the  observations  of  E.  Wilson  and  Frost  (note  to  p.  170). 


SUN-SPOT  THEORIES.  179 

already  mentioned,  as  well  as  that  of  Sir  William 
Herschel,  which  we  must  now  present,  all  proceed 
upon  the  assumption  that  the  central  core  of  the  sun 
is  solid. 

About  the  beginning  of  the  present  century,  Sir 
William  Herschel,  after  a  careful  study  of  the  facts, 
but  much  influenced  by  the  belief  that  the  sun  must 
(for  theological  reasons)  be  a  habitable  body,  proposed 
an  hypothesis  which  stood  unchallenged  for  nearly  half 
a  century. 


FIG.  43. 


PHOTO-SPHERE:. 

POIUMBRAL  CLOUD. 
BODY  OF  SUN. 


W.  HBRSCHEL'S  SUN-SPOT  THKOBY. 


He  supposed  the  central  portion  of  the  sun  to  be 
solid ;  its  surface  cool,  non-luminous,  and  habitable. 
Around  this  he  placed  two  envelopes  of  cloud — the 
outer  one,  the  photosphere,  incandescent,  blazing  with 
unimaginable  fury ;  the  inner  one  non-luminous,  dark 
itself,  but  capable  of  reflecting  light  from  its  upper  sur- 
face, and  acting  as  a  screen  to  protect  the  underlying 
country  from  the  heat  of  the  photosphere.  The  spots 
he  supposed  to  be  caused  by  temporary  openings  in 
the  clouds,  through  which  we  could  look  down  upon 
the  dark  surface  of  the  central  globe  ;  the  penumbra 
being  caused  by  the  intermediate  cloud-layer,  opening 
less  widely  than  the  photosphere.  The  figure  illustrates 
this  theory.  As  to  the  cause  of  the  openings  he  uttered 
no  decided  opinion,  though  suggesting  that  they  might 
be  due  to  volcanic  eruptions,  forcing  their  way  up 
through  the  higher  atmosphere. 

His  son,  Sir  John  Herschel,  many  years  later,  pro- 


180  THE  SUN. 

posed  an  explanation  which  would  make  the  spots  to  be 
great  whirling  storms  boring  down  through  the  photo- 
sphere and  clouds,  instead  of  eruptions  pushing  their 
way  outward.  According  to  him,  the  rotation  of  the 
sun  causes  an  accumulation  of  the  solar  atmosphere  at 
the  sun's  equator — a  thickening  of  the  layer  which  ob- 
structs the  radiation  of  heat.  This  being  so,  there  should 
be  on  the  sun,  as  on  the  earth,  though  for  an  entirely 
different  reason,  a  temperature  higher  in  the  equatorial 
regions  than  elsewhere ;  and  then  would  follow  a  long 
train  of  consequences,  among  them  these  :  the  solar  at- 
mosphere would  be  disturbed  by  currents  like  the  trade- 
winds  on  the  earth ;  there  would  be  stormy  zones  on 
each  side  the  equator,  and  these  storms  would  furnish 
an  explanation  of  the  spots. 

To  a  certain  extent,  the  cause  adduced  must  actually 
exist.  The  sun's  rotation  must  necessarily  thicken  the 
atmospheric  layer  which  overlies  the  photosphere  (i.  e., 
it  must,  if  the  surfaces  of  the  photosphere  and  chromo- 
sphere can  be  regarded  as  level  surfaces),  and  this  cause 
must  tend  to  raise  the  actual  temperature  of  the  sun's 
equator,  while  at  the  same  time  it  must  diminish  its 
radiation  to  the  earth,  and  so  render  the  solar  equator 
apparently  cooler,  as  tested  by  our  observations  from 
the  earth.  But,  so  far  as  can  be  judged,  this  effect  is 
<^uite  insensible,  as  it  should  be,  since  the  sun's  rotation 
is  so  slow ;  and  the  motions  of  the  spots  show  no  such 
systematic  drift  north  or  south  as  solar  trade-winds 
would  necessarily  produce. 

The  elder  Herschel's  theory  satisfies  all  the  tele- 
Bcopic  appearances  of  sun-spots  quite  as  well,  perhaps, 
as  any  yet  proposed.  It  breaks  down  in  its  assumption 
that  the  principal  portion  of  the  sun  is  a  solid  mass,  an 
assumptioD  which  is  now  almost  universally  regarded 


SUN-SPOT  THEORIES. 


181 


as  incompatible  with  what  we  know  of  the  soiar  tem- 
perature, radiation,  and  constitution. 

It  seems  to  modern  physicists  an  unavoidable  con- 
clusion that  the  sun's  central  mass  must  be  gaseous,  or 
at  least  not  solid.  Setting  out  with  this  idea,  Faye  and 
Secchi  independently,  about  1868,  proposed  the  theory 
that  the  spots  are  openings  in  the  photosphere,  through 


FIG.  44. 


SECCHI'S  FIRST  SPOT-THEORY. 

which  the  internal  gases  are  bursting  outward.  We 
present  one  of  SecchPs  figures  illustrating  this  view. 
But  it  was  abandoned  by  its  proposers  as  soon  as  it  was 
clearly  pointed  out  that  in  that  case  the  spectrum  of 
the  umbra  of  a  sun-spot  should  be  composed  of  bright 
lines ;  and  Secchi  himself  and  others  had  shown  that  it  is 
not  so  at  all,  but  a  spectrum  due  to  increased  absorption, 


182  THE  SUN. 

and  probably  indicating,  not  an  up-rush  of  heated  gases 
through  the  photosphere,  but  a  descent  of  cooler 
and  less  luminous  matter  from  above.  In  this  connec- 
tion the  observations  of  the  writer  and  of  Duner  may 
be  referred  to  (page  132).  But  the  theory  has  great 
vitality.  Mr.  Proctor,  in  his  "  Old  and  New  Astron- 
omy," maintained  it,  and  it  continually  turns  up  in 
the  speculations  of  popular  writers.  About  1870  Zoll- 
ner  proposed  a  peculiar  theory  which  has  many  good 
points  about  it,  but  seems  obnoxious  to  fatal  objec- 
tions, and  has  found  very  few  defenders.  He  conceives 
the  surface  of  the  sun  to  be  liquid — a  molten  mass  over- 
laid by  an  atmosphere  of  vapor.  This  liquid  surface  he 
imagines  to  be  here  and  there  covered  at  times  by  slag- 
like  masses  of  much  lower  radiating  power,  the  result 
of  local  cooling.  Around  their  edges  the  solar  flames 
burst  out  with  redoubled  fury,  but  at  the  center  the 
cooler  mass  of  scoria  determines  a  downward  current, 
so  as  to  establish  a  powerful  circulation  in  the  solar  at- 
mosphere— downward  at  the  center  of  the  spot,  outward 
in  all  directions  at  the  surface  of  the  slag,  upward  all 
around  its  margin,  and  inward,  toward  the  center,  in 
the  upper  air.  This  theory  admirably  agrees  with  the 
spectroscopic  phenomena ;  but  the  hypothesis  of  a  con- 
tinuous liquid  shell,  cool  enough  to  permit  the  forma 
tion  of  scoriae,  seems  inconsistent  with  other  phenom- 
ena, which  make  it  impossible  to  admit  so  low  a  tem- 
perature at  so  great  a  depth. 

At  present,  opinion,  for  the  most  part,  seems  to  be 
divided  between  two  rival  theories  proposed  by  Faye 
and  Secchi. 

Faye  conceives  the  sun-spots  to  be  the  effect  of 
solar  storms ;  Secchi  believes  them  to  be  dense  clouds 
of  eruption-products  settling  down  into  the  photo- 


SUN-SPOT   THEORIES.  183 

sphere  near,  but  not  at,  the  points  where  they  were 
ejected. 

Faye,  it  will  be  remembered,  supposes  the  sun's  pe- 
culiar law  of  rotation  to  be  due  to  the  hypothetical  fact 
that  the  ascending  masses  of  vapors  (which  form  the 
photosphere  by  their  condensation)  start  from  a  stratum 
whose  depth  below  the  visible  surface  regularly  dimin- 
ishes from  the  equator  toward  the  poles.  Hence  re- 
sult currents  parallel  to  the  equator,  and  the  conse- 
quence is  that,  generally  speaking,  neighboring  portions 
of  the  photosphere  have  a  relative  drift.  At  the  equa- 
tor and  at  the  poles  this  drift  vanishes,  but  is  most  con- 
siderable in  the  middle  latitudes.  Now,  it  is  Faye's 
theory  that,  in  consequence  of  this  relative  drift,  eddies 
are  formed,  as  explained  on  a  preceding  page ;  these 
eddies  become  cyclones  or  whirls  precisely  analogous 
to  those  seen  in  water  where  a  rapid  current  is  obstruct- 
ed by  an  obstacle.  In  such  a  case,  as  every  one  knows, 
tunnel-shaped  vortices  are  formed,  down  which  floating 
materials  and  air  are  carried  to  considerable  depths. 
Our  terrestrial  whirlwinds  and  tornadoes  are  produced, 
according  to  Faye  (but  in  opposition  to  the  generally 
received  theories),  in  a  similar  manner,  beginning  from 
above,  and  penetrating  downward  until  the  point  of  the 
whirling  vortex  reaches  and  sweeps  the  earth.  Now, 
such  a  vortex,  on  the  solar  scale,  is  the  essence  of  a  sun- 
spot,  according  to  Faye. 

It  is  evident  at  once  that  this  theory  gives  a  reason- 
able explanation  of  the  distribution  of  the  spots  in  two 
parallel  zones  on  each  side  of  the  sun's  equator,  and  that 
the  drifting  action,  in  which  the  cause  of  the  spots  is 
supposed  to  lie,  is  a  vera  causa. 

The  theory  accords  very  well,  also,  with  the  phe- 
nomena which  accompany  the  subdivision  of  spots, 


184  THE  SUN. 

since  whirls  in  water  and  cyclones  in  the  terrestrial 
atmosphere  behave  in  precisely  the  same  sort  of  way. 
It  fairly  meets,  too,  the  spectroscopic  indications.  The 
cavity  filled  with  descending  vapors  would  naturally 
give  just  such  a  kind  of  spectrum  as  that  which  is  ordi- 
narily observed.  Moreover,  the  gases  carried  down  in 
the  vortex  below  the  photosphere,  especially  the  hydro- 
gen, would  boil  up  again  all  around  the  whirlpool,  and 
thus  we  could  account  for  the  ring  of  faculae  and  prom- 
inences which,  as  a  general  rule,  environs  every  spot  of 
considerable  magnitude.  Some  of  the  more  obvious 
objections  can  also  be  easily  disposed  of.  Thus,  it 
has  been  said  that,  if  the  sun-spots  are  such  vortices, 
they  ought  to  be  circular  in  outline.  Faye  replies  that 
we  see,  not  the  vortex  itself,  but  a  great  cloud  of  cooler 
gases,  sucked  down  from  above  and  gathered  into  the 
storm  from  all  sides,  and  the  form  of  this  cloud  would 
depend  upon  a  multitude  of  circumstances. 

But  there  are  other  objections  which  are  not  so  easily 
met.  It  the  theory  be  true,  all  spots  are  whirls  and 
ought  to  show  a  vortical  motion,  and,  what  is  more,  all 
spots  north  of  the  equator  ought  to  whirl  in  the  same 
direction,  and  against  the  hands  of  a  watch  (as  seen 
from  the  earth),  while  those  in  the  sun's  southern  hemi- 
sphere should  revolve  in  the  contrary  direction,  pre- 
cisely as  cyclones  do  in  the  atmosphere  of  the  earth. 

Now,  this  is  not  the  case  at  all.  As  we  have  seen, 
only  a  very  small  percentage  of  the  spots  show  any 
trace  of  vorticose  motion*;  and,  so  far  from  observing 
any  uniformity  in  the  direction  of  rotation  on  each  side 
of  the  equator,  we  frequently  find  different  members  of 
the  same  group  of  spots,  or  even  different  portions  of 
the  self-same  spot,  revolving  oppositely. 

In  fact,  when  we  come  to  look  into  the  matter  nu» 


SUN-SPOT  THEORIES,  j  185 

merically,  we  find  that  the  drift,  which  Faye  makes 
the  determining  factor  of  sun-spot  genesis,  is  far  too 
slight  to  produce  such  effects. 

It  is  very  easy  to  compute  this  drift  if  we  assume 
the  correctness  of  Faye's  own  formula  for  the  motion 
of  a  point  on  the  sun's  surface  in  any  given  solar  lati- 
tude, viz.,  V'  =  862'  —  186' sin1  X;  V  in  this  formula 
being  the  number  of  minutes  of  solar  longitude  passed 
over  by  any  given  point  in  twenty-four  hours. 

If  we  apply  this  formula  to  two  points  on  the  solar 
surface,  one  in  latitude  20°  and  the  other  in  latitude 
20°  1',  i,  e.,  about  123  miles  north  of  the  first,  we  shall 
find  that  the  first  has  a  daily  motion  of  840-242'  and 
the  second  840-207',  a  difference  of  only  -035',  or  (in 
this  latitude)  4- IT  miles.  That  is  to  say,  if  we  take  two 
points  on  the  solar  surface,  on  the  same  meridian,  in 
latitude  20°,  at  a  distance  of  123  miles,  the  one  nearer 
the  equator  will,  at  the  end  of  twenty-four  hours,  have 
drifted  about  4£  miles  to  the  eastward  of  the  other. 

If  we  make  the  same  calculation  for  latitude  45°, 
we  get  a  result  a  trifle  greater — about  4rJ  miles  per  day. 

With  these  figures  it  is  easy  to  see  why  the  sun-spots 
do  not  behave  more  like  the  disturbances  of  our  terres- 
trial atmosphere,  in  exhibiting  cyclonic  motion  as  a 
regular  and  invariable  characteristic,  instead  of  an  occa- 
sional and  rather  a  rare  phenomenon. 

Secchi's  latest  theory  is  based  essentially  upon  the 
idea,  certainly  borne  out  by  observation,  that  eruptions 
are  continually  breaking  through  the  photosphere,  and 
carrying  up  metallic  vapors  from  the  regions  beneath. 
He  imagines  that  these  vapors,  after  becoming  consid- 
erably cooled,  descend  upon  the  photosphere  and  form 
depressions  in  it,  which  are  filled  with  these  less  lumi- 
nous and  absorbent  materials.  It  is  difficult  to  see  why 


18(5  THE  SUN. 

the  effect  should  remain  so  persistent,  or  why,  even  if 
the  eruption  be  long  maintained,  the  cloud  should  con- 
tinue to  descend  in  the  same  place.  In  fact,  as  was 
said  only  a  few  moments  ago,  a  spot  is  generally  sur- 
rounded by  a  ring  of  eruptions,  and  things  take  place 
as  if  they  were  all  pouring  their  ejections  into  the  same 
receptacle — as  if  there  were,  in  fact,  some  such  down- 
ward suction  through  the  center  of  the  spot  as  the  the- 
ory of  Faye  supposes,  an  aspiration  capable  of  drawing 
in  toward  the  spot  all  erupted  materials  in  the  vicinity. 
The  sun-spot  theories  of  Lockyer  and  Schaeberle, 
already  referred  to  on  page  143  in  connection  with  the 
explanation  of  the  equatorial  acceleration  of  the  sun's 
rotation,  agree  with  this  theory  of  Secchi's  in  attributing 
the  spots  to  the  downfall  of  matter  from  a  great  ele- 
vation. Schaeberle  supposes  it  to  be  matter  simply 
blown  out  by  eruptions,  some  of  it  with  force  enough 
to  carry  it  out  even  to  the  orbits  of  Jupiter  and  Saturn ; 
on  its  return  it  penetrates  and  chills  the  photosphere. 
Mr.  Lockyer,  if  we  understand  him  rightly,  in  his  sug- 
gestions which  form  the  closing  chapter  of  the  "  Chem- 
istry of  the  Sun,"  is  rather  disposed  to  think  that  the 
"  iron  "  and  such  other  substances  as  by  their  fall  pro- 
duce the  spots  are  formed  by  the  union  and  combina- 
tion of  their  elementary  constituents,  which  have  as- 
cended in  a  "  dissociated  "  condition  to  the  upper  regions 
of  the  solar  atmosphere.  There,  where  the  temperature 
is  no  longer  above  the  "  dissociation  "  point,  the  atoms 
recombine  into  molecules  of  iron-vapor,  etc. ;  the  vapors 
condense  into  clouds  and  liquid  masses,  and  these  de- 
scend upon  the  photosphere.  They  absorb  heat  all  the 
way  down,  by  revaporization  and  new  dissociation  chill- 
ing the  photosphere  where  they  pierce  it  and  causing  a 
"  splash  "  or  up-rush  of  the  photospheric  matter  and  its 


SUN-SPOT   THEORIES.  187 

underlying  gases  all  around  the  spot,  which  we  recog- 
nize as  faculae,  prominences  and  metallic  eruptions. 
According  to  these  theories  the  faculse  and  eruptions 
are  consequences  of  the  formation  of  the  spot :  accord- 
ing to  Secchi  they  precede  and  cause  it.  It  would  seein 
easy  to  decide  the  question  by  observation,  but  it  does 
not  appear  to  be  so;  on  the  whole,  however,  the  weight 
of  evidence  is  pretty  strongly  in  favor  of  the  opinion 
that  faculae  and  pores  and  a  general  disturbance  of  the 
region  are  usually  obvious  before  the  spot  manifests  it- 
self ;  and  it  must  be  admitted  that  in  some  cases  the 
appearances  puzzlingly  resemble  the  emergence  of  a 
dark  inass/TWw.  beneath. 

Probably  both  Lockyer  and  Schaeberle  would  cheer- 
fully accept  Sir  John  Herschel's  theory  to  a  certain  ex- 
tent— that  some  of  the  spots  may  be  due  to  the  fall 
upon  the  sun  of  great  meteors  from  outer  space.  While 
it  is  hardly  possible  that,  directly r,  a  meteor,  such  as  we 
know  meteors  upon  the  earth,  could  by  its  fall  produce 
even  a  small  sun-spot,  it  is  not  easy  to  say  what  might 
be  the  indirect  effects  consequent  upon  its  passage 
through  the  photosphere,  and  its  disturbance  of  the 
dynamical  equilibrium. 

The  writer  some  time  ago  suggested  a  modification 
of  Secchi's  theory,  which  seems  to  remove  some  of  the 
objections,  and  appears  on  the  whole  more  probable 
than  any  of  the  others.  It  may  be  that  the  spots  are 
depressions  in  the  photospheric  level,  caused  not  directly 
by  the  pressure  of  the  erupted  materials  from  above, 
but  by  the  diminution  of upward  pressure  from  below, 
in  consequence  of  eruptions  in  the  neighborhood ;  the 
spots  thus  being,  so  to  speak,  sinks  in  the  photosphere. 
Undoubtedly  the  photosphere  is  not  a  strictly  continu- 
ous shell  or  crust,  but  it  is  heavy  as  compared  with  the 


188  THE  SUN. 

uncondensed  vapors  in  which  it  lies,  just  as  a  rain-cloud 
in  our  terrestrial  atmosphere  is  heavier  than  the  air, 
and  it  is  probably  continuous  enough  to  have  its  upper 
level  affected  by  any  diminution  of  pressure  below, 
The  gaseous  mass  below  the  photosphere  supports  its 
weight  and  the  weight  of  the  products  of  condensation, 
which  must  always  be  descending  in  an  inconceivable 
rain  and  snow  of  molten  and  crystallized  material.  To 
all  intents  and  purposes,  though  nothing  but  a  layer  of 
clouds,  the  photosphere  thus  forms  a  constricting  shell, 
and  the  gases  beneath  are  imprisoned  and  compressed. 
Moreover,  at  a  high  temperature  the  viscosity  of  gases 
is  vastly  increased,  so  that  quite  probably  the  matter  of 
the  solar  nucleus  resembles  pitch  or  tar  in  its  consist- 
ency more  than  what  we  usually  think  of  as  a  gas. 
Consequently,  any  sudden  diminution  of  pressure  would 
propagate  itself  gradually  from  the  point  where  it  oc- 
curred. Putting  these  things  together,  it  would  seem 
that,  whenever  a  free  outlet  is  obtained  through  the 
photosphere  at  any  point,  thus  decreasing  the  inward 
pressure,  the  result  would  be  the  sinking  of  a  portion 
of  the  photosphere  somewhere  in  the  immediate  neigh- 
borhood, to  restore  the  equilibrium ;  and,  if  the  erup- 
tion were  kept  up  for  any  length  of  time,  the  depression 
in  the  photosphere  would  continue  till  the  eruption 
ceased.  This  depression,  filled  with  the  overlying  gases, 
would  constitute  a  spot.  Moreover,  the  line  of  frac- 
ture, if  we  may  call  it  so,  at  the  edges  of  the  sink  would 
be  a  region  of  weakness  in  the  photosphere,  so  that  we 
should  expect  a  series  of  eruptions  all  around  the  spot. 
For  a  time  the  disturbance,  therefore,  would  grow,  and 
the  spot  would  enlarge  and  deepen,  until,  in  spite  of 
the  viscosity  of  the  internal  gases,  the  equilibrium  of 
pressure  was  gradually  restored  beneath.  So  far  as  we 


SUN-SPOT  THEORIES.  189 

know  the  spectroscopic  and  visual  phenomena,  none  of 
them  contradict  this  hypothesis. 

As  regards  the  limitation  of  the  spots  to  certain 
latitudes,  this,  as  has  been  said  already,  almost  certainly 
will  find  its  explanation  in  that  of  the  equatorial  accel- 
eration. Faye,  Belopolsky,  Lockyer,  and  Schaeberle, 
all  present  such  explanations.  Schaeberle's  discussion 
of  the  subject  may  be  found  in  "  Astronomy  and  Astro- 
Physics,"  for  April,  1894.  We  shall  have  occasion  to 
refer  to  it  again  in  connection  with  the  corona. 

"Whatever  may  be  the  cause  of  spots,  it  is  probable 
that  the  annexed  figure  gives  a  fair  idea  of  the  arrange- 
ment and  relations  of  the  photospheric  clouds  in  the 

FIG.  45. 


CONSTITUTION  OF  A  SUN-SPOT. 


neighborhood  of  one.  Over  the  sun's  surface  gener- 
ally, these  clouds  probably  have  the  form  of  vertical 
columns,  as  at  a  a.  Just  outside  the  spot,  the  level  of 
the  photosphere  is  usually  raised  into  faculae,  as  at  b  b. 
These  faculae  are  for  the  most  part  overtopped  by 
eruptions  of  hydrogen  and  metallic  vapors,  as  indicated 
by  the  shaded  clouds.  Of  these  metallic  eruptions  we 
shall  have  more  to  say  in  the  chapter  upon  the  chromo- 
sphere and  prominences,  only  remarking  here  that, 


190  THE 

while  the  great  clouds  of  hydrogen  are  found  every- 
where upon  the  sun,  these  spiky,  vivid  outbursts  of 
metallic  vapors  seldom  occur,  except  just  in  the  neigh- 
borhood of  a  spot,  and  then  only  during  its  season  of 
rapid  change.  In  the  penumbra  of  the  spot  the  photo- 
spheric  filaments  become  more  or  less  nearly  horizontal, 
as  atpp  •  in  the  umbra,  at  u,  it  is  quite  uncertain  what 
the  true  state  of  affairs  may  be.  "We  have  conjecturally 
represented  the  filaments  there  as  vertical  also,  but  de- 
pressed and  carried  down  by  a  descending  current.  Of 
course,  the  cavity  o  o  is  filled  by  the  gases  which  overlie 
the  photosphere ;  and  it  is  easy  to  see  that,  looked  at 
from  above,  such  a  cavity  and  arrangement  of  the 
luminous  filaments  would  present  the  appearances  actu- 
ally observed. 

Oppolzer,  of  Vienna,  in  1893  proposed  a  new  theory 
based  largely  upon  Hann's  researches  on  the  tempera^ 
ture  effects  of  vertical  atmospheric  currents.  Such  cur- 
rents are  supposed  to  rise  periodically  from  the  polar 
regions  of  the  sun,  to  drift  slowly  toward  its  equator, 
and  to  descend  in  the  spot  zones,  'becoming  heated  and 
"dried"  in  their  descent,  thus  forming  in  the  photo- 
sphere cavities  which  are  filled  with  metallic  vapors  in 
purely  gaseous  condition. 

In  many  ways  the  theory  admirably  corresponds 
with  facts,  explaining  better  than  any  other  the  peculiar 
character  of  the  sun-spot  spectrum,  Spoerer's  law  of 
sun-spot  latitudes,  and  the  otherwise  puzzling  observa- 
tions of  Langley  and  Frost  upon  sun-spot  temperatures 
(page  170,  note).  But  the  polar  streams  themselves  are 
unaccounted  for,  and  it  remains  to  be  seen  how  this 
"meteorological"  theory  will  withstand  other  adverse 
criticisms, 


CHAPTER  VI. 

THE  CHROMOSPHERE  AND  THE  PROMINENCES. 

Early  Observations  of  Chromosphere  and  Prominences. — The  Eclipses  of 
1842,  1851,  and  I860.— The  Eclipse  of  1868.— Discovery  of  Janssen 
and  Lockyer. — Arrangement  of  Spectroscope  for  Observations  upon 
Chromosphere. — Spectrum  of  Chromosphere. — Lines  always  present. 
— Lines  often  reversed. — Ultra- Violet  Studies  of  Hale  and  Des- 
landres. — Motion  Forms. — Double  Reversal  of  Lines. — Distribution 
of  Prominences. — Magnitude. — Classification  of  Prominences  as  qui- 
escent, and  eruptive  or  metallic. — Isolated  Clouds. — Violence  of  Mo- 
tion.— Observations  of  August  5,  1872. — Theories  as  to  the  Forma- 
tion and  Causes  of  the  Prominences. 

WHAT  we  see  of  the  sun  under  ordinary  circum- 
stances is  but  a  fraction  of  his  total  bulk.  While  by 
far  the  greater  portion  of  the  solar  mass  is  included 
within  the  photosphere — the  blazing  cloud-layer,  which 
seems  to  form  the  sun's  true  surface,  and  is  the  princi- 
pal source  of  his  light  and  heat — yet  the  larger  portion 
of  his  volume  lies  without,  and  constitutes  an  atmos- 
phere whose  diameter  is  at  least  double,  and  its  bulk 
therefore  sevenfold  that  of  the  central  globe. 

Atmosphere,  however,  is  hardly  the  proper  term ; 
for  this  outer  envelope,  though  gaseous  in  the  main,  is 
not  spherical,  but  has  an  outline  exceedingly  irregular 
and  variable.  It  seems  to  be  made  up  not  of  overlying 
strata  of  different  density,  but  rather  of  flames,  beams, 
and  streamers,  as  transient  and  unstable  as  those  of  our 
own  aurora  borealis.  It  is  divided  into  two  portions, 
separated  by  a  boundary  as  definite,  though  not  so 


192  THE 

regular,  as  that  which  parts  them  both  from  the  photo- 
sphere. The  outer  and  far  more  extensive  portion, 
which  in  texture  and  rarity  seems  to  resemble  the  tails 
of  comets,  and  may  almost,  without  exaggeration,  be 
likened  to  "  the  stuff  that  dreams  are  made  of,"  is 
known  as  the  "  coronal  atmosphere,"  since  to  it  is 
chiefly  due  the  "  corona  "  or  glory  which  surrounds  the 
darkened  sun  during  an  eclipse,  and  constitutes  the 
most  impressive  feature  of  the  occasion. 

At  its  base,  and  in  contact  with  the  photosphere,  is 
what  resembles  a  sheet  of  scarlet  fire.  The  appearance, 
which  probably  indicates  a  fact,  is  as  if  countless  jets 
of  heated  gas  were  issuing  through  vents  and  spiracles 
over  the  whole  surface,  thus  clothing  it  with  flame 
which  heaves  and  tosses  like  the  blaze  of  a  conflagra- 
tion— "  like  a  prairie  on  fire,"  to  quote  the  vividly  de- 
scriptive phrase  of  Professor  Langley. 

This  has  received  the  name  of  chromosphere,  a 
designation  first  proposed  by  Frankland  and  Lockyer 
in  1869,  and  signifying  "  color-sphere,"  in  allusion  to 
the  vivid  redness  of  the  stratum,  caused  by  the  pre- 
dominance of  hydrogen  in  these  flames  and  clouds.  It 
was  called  the  "  sierra  "  by  Airy  in  1842,  and  Proctor 
and  some  other  writers  prefer  that  name  to  the  later 
and  more  common  appellation. 

Here  and  there  masses  of  this  hydrogen  mixed  with 
other  substances  rise  to  a  great  height,  ascending  far 
above  the  general  level  into  the  coronal  regions,  where 
they  float  like  clouds,  or  are  torn  to  pieces  by  contend- 
ing currents.  These  cloud-masses  are  known  as  solar 
"prominences,"  or  "protuberances,"  a  non-committal 
sort  of  appellation  applied  in  1842,  when  they  first 
attracted  any  considerable  attention,  and  while  it  was 
a  warmly-disputed  question  whether  they  were  solar, 


THE  CHROMOSPHERE  AND  THE  PROMINENCES.      193 

lunar,  phenomena  of  our  own  atmosphere,  or  even  mere 
optical  illusions.  It  is  unfortunate  that  no  more  appro- 
priate and  graphic  name  has  yet  been  found  for  objects 
of  such  wonderful  beauty  and  interest. 

Until  recently,  the  solar  atmosphere  could  be  seen 
only  at  an  eclipse,  when  the  sun  itself  is  hidden  by  the 
moon.  Now,  however,  the  spectroscope  has  brought 
the  chromosphere  and  the  prominences  within  the  range 
of  daily  observation,  so  that  they  can  be  studied  with 
nearly  the  same  facility  as  the  spots  and  faculse,  and 
a  fresh  field  of  great  interest  and  importance  is  thus 
opened  to  science. 

It  seems  hardly  possible  that  the  ancients  should 
have  failed  to  notice,  even  with  the  naked  eye,  in  some 
one  of  the  many  eclipses  on  record,  the  presence  of 
blazing,  star-like  objects  around  the  edge  of  the  moon, 
but  we  find  no  mention  of  any  thing  of  the  kind,  al- 
though the  corona  is  described  as  we  see  it  now.  On 
this  ground  some  have  surmised  that  the  sun  has  really 
undergone  a  change  in  modern  times,  and  that  the 
chromosphere  and  prominences  are  a  new  development 
in  the  solar  history.  But  such  mere  negative  evidence 
is  altogether  insufficient  as  a  foundation  for  so  impor- 
tant a  conclusion. 

The  earliest  recorded  observation  of  the  prominences 
is  probably  that  of  Yassenius,  a  Swedish  astronomer, 
who,  during  the  total  eclipse  of  1T33,  noticed  three  or 
four  small  pinkish  clouds,  entirely  detached  from  the 
limb  of  the  moon,  and,  as  he  supposed,  floating  in  the 
lunar  atmosphere.  At  that  time  this  was  the  most 
natural  interpretation  of  the  appearance,  since  the  fact 
that  the  moon  has  no  atmosphere  was  not  yet  ascer- 
tained. 

The  Spanish  admiral,  Don  Ulloa,  in  his  account  of 
14 


194  THE   SUN. 

the  eclipse  of  1778,  describes  a  point  of  red  light  which 
made  its  appearance  on  the  western  limb  of  the  moon 
about  a  minute  and  a  quarter  before  the  emergence  of 
the  sun.  At  first  small  and  faint,  it  grew  brighter  and 
brighter  until  extinguished  by  the  returning  sunlight. 
He  supposed  that  the  phenomenon  was  caused  by  a 
hole  or  fissure  in  the  body  of  the  moon ;  but,  with  our 
present  knowledge,  there  can  be  little  doubt  that  it  was 
simply  a  prominence  gradually  uncovered  by  her  motion. 

The  chromosphere  seems  to  have  been  seen  even 
earlier  than  the  prominences :  thus  Captain  Stannyan, 
in  a  report  on  the  eclipse  of  1706,  observed  by  him  at 
Berne,  noticed  that  the  emersion  of  the  sun  was  pre- 
ceded by  a  blood-red  streak  of  light,  visible  for  six  or 
seven  seconds  upon  the  western  limb.  Halley  and 
Louville  saw  the  same  thing  in  1715.  Halley  says  that 
two  or  three  seconds  before  the  emersion  a  long  and 
very  narrow  streak  of  a  dusky  but  strong  red  light 
seemed  to  color  the  dark  edge  of  the  moon  on  the 
western  edge  where  the  sun  was  about  to  reappear. 
Louville's  account  agrees  substantially  with  this,  and 
he  further  describes  the  precautions  he  used  to  satisfy 
himself  that  the  phenomenon  was  no  mere  optical  illu- 
sion, nor  due  to  any  imperfection  of  his  telescope. 

In  eclipses  that  followed  that  of  1733,  the  chromo- 
sphere and  prominences  seem  to  have  attracted  but  lit- 
tle attention,  even  if  they  were  observed  at  all.  Some- 
thing of  the  sort  appears  to  have  been  noticed  by  Ferrers 
in  1806,  but  the  main  interest  of  his  observation  lay  in 
a  different  direction. 

In  July,  1842,  a  great  eclipse  occurred,  and  the 
shadow  of  the  moon  described  a  wide  belt  running 
across  southern  France,  northern  Italy,  and  a  portion 
of  Austria.  The  eclipse  was  carefully  observed  by 


THE   CHROMOSPHERE  AND   THE   PROMINENCES.      195 

many  of  the  most  noted  astronomers  of  the  world,  and 
so  completely  had  previous  observations  of  the  kind 
been  forgotten,  that  the  prominences,  which  appeared 
then  with  great  brilliance,  were  regarded  with  extreme 
surprise,  and  became  objects  of  warm  discussion,  not 
only  as  to  their  cause  and  location,  but  even  as  to  their 
very  existence.  Some  thought  them  mountains  upon 
the  sun,  some  that  they  were  solar  flames,  and  others, 
clouds  floating  in  the  sun's  atmosphere.  Others  re- 
ferred them  to  the  moon,  and  yet  others  claimed  that 
they  were  mere  optical  illusions.  At  the  eclipse  of 
1851  (in  Sweden  and  Norway),  similar  observations 
were  repeated,  and,  as  a  result  of  the  discussions  and 
comparison  of  observations  which  followed,  astronomers 
generally  became  satisfied  that  the  prominences  are  real 
phenomena  of  the  solar  atmosphere,  in  many  respects 
analogous  to  our  terrestrial  clouds;  and  several  came 
more  or  less  confidently  to  the  conclusion,  now  known 
to  be  true  (see  Grant's  "  History  of  Physical  Astrono- 
my "),  that  the  sun  is  entirely  surrounded  with  a  con- 
tinuous stratum  of  the  same  substance.  Many,  how- 
ever, remained  unconvinced :  Faye,  for  instance,  still 
asserted  them  to  be  mere  optical  illusions,  or  mirages. 

In  the  eclipse  of  1860,  photography  was  for  the  first 
time  employed  on  such  an  occasion  with  anything  like 
success.  The  results  of  Secchi  and  De  La  Rue  removed 
all  remaining  doubts  as  to  the  real  existence  and  solar 
character  of  the  objects  in  question,  by  exhibiting  them 
upon  their  plates  gradually  covered  on  one  side  and  un- 
covered on  the  other  side  of  the  sun  by  the  progress  of 
the  moon. 

Secchi  thus  sums  up  his  conclusions,  which  have 
been  justified  in  almost  all  their  details  by  later  obser- 
vations ;  they  require  few  and  slight  corrections : 


196  THE  SUN. 

"  1,  The  prominences  are  not  mere  optical  illusions ; 
they  are  real  phenomena  pertaining  to  the  sun.  .  .  . 

2.  The  prominences  are  collections  of  luminous  mat- 
ter of  great  brilliance,  and  possessing  remarkable  pho- 
tographic activity.     This  activity  is  so  great  that  many 
of  them,  which  are  visible  in  our  photographs,  could 
not  be  seen  directly  even  with  good  instruments. 

3.  Some  protuberances  float  entirely  free  in  the  so- 
lar atmosphere  like  clouds.    If  they  are  variable  in  form, 
their  changes  are  so  gradual  as  to  be  insensible  in  the 
space  of  ten  minutes.     (Generally,  but  by  no  means  al- 
ways, true.) 

4.  Besides  the  isolated  and  conspicuous  protuber- 
ances there  is  also  a  layer  of  the  same  luminous  sub- 
stance which  surrounds  the  whole  sun,  and  out  of  which 
the  protuberances  rise  above  the  general  level  of  the  so- 
lar surface.  .  .  . 

5.  The  number  of  the  protuberances  is  indefinitely 
great.     In  direct  observation  through  the  telescope  the 
sun  appeared  surrounded  with  flames  too  numerous  to 
count.  .  .  . 

6.  The  height  of  the  protuberances  is  very  great, 
especially  when  we  take  account  of  the  portion  hidden, 
by  the  moon.     One  of  them  had  a  height  of  at  least 
three  minutes,  which  indicates  a  real  altitude  of  more 
than  ten  times  the  earth's  diameter.  ..." 

But  their  nature  still  remained  a  mystery ;  and  no 
one  could  well  be  blamed  for  thinking  it  must  always 
remain  so  to  some  degree.  At  that  time  it  could  hard- 
ly be  hoped  that  we  should  ever  be  able  to  ascertain 
their  chemical  constitution,  and  measure  the  velocities 
of  their  motions.  And  yet  this  has  been  done.  Before 
the  great  Indian  eclipse  of  August  18,  1868,  the  spec- 
troscope had  been  invented  (it  was,  indeed,  already  in 


THE   CHROMOSPHERE  AND  THE  PROMINENCES. 

its  infancy  in  1860),  and  applied  to  astronomical  research 
with  the  most  astonishing  and  important  results. 

Every  one  is  more  or  less  familiar  with  the  story  of 
this  eclipse.  Herschel,  Tennant,  Pogson,  Eayet,  and 
Janssen,  all  made  substantially  the  same  report.  They 
found  the  spectrum  of  the  prominences  observed  to  con- 
sist of  bright  lines,  and  conspicuous  among  them  were 
the  lines  of  hydrogen.  There  were  some  serious  dis- 
crepancies, indeed,  among  their  observations,  not  only 
as  to  the  number  of  the  bright  lines  seen,  which  is  not 
to  be  wondered  at,  but  as  to  their  position.  Thus, 
Kayet  (who  saw  more  lines  than  any  one  else)  identified 
the  red  line  observed  with  B  instead  of  C ;  and  all  the 
observers  mistook  the  yellow  line  they  saw  for  that  of 
sodium. 

Still,  their  observations,  taken  together,  completely 
demonstrated  the  fact  that  the  prominences  are  enor- 
mous masses  of  highly-heated  gaseous  matter,  and  that 
hydrogen  is  a  main  constituent. 

Janssen  went  further.  The  lines  he  saw  during  the 
eclipse  were  so  brilliant  that  he  felt  sure  he 'could  see 
them  again  in  the  full  sunlight.  He  was  prevented  by 
clouds  from  trying  the  experiment  the  same  afternoon, 
after  the  close  of  the  eclipse ;  but  the  next  morning  the 
sun  rose  unobscured,  and,  as  soon  as  he  had  completed 
the  necessary  adjustments,  and  directed  his  instrument 
to  the  portion  of  the  sun's  limb  where  the  day  before 
the  most  brilliant  prominence  appeared,  the  same  lines 
came  out  again,  clear  and  bright ;  and  now,  of  course, 
there  was  no  difficulty  in  determining  at  leisure,  and 
with  almost  absolute  accuracy,  their  position  in  the 
spectrum.  He  immediately  confirmed  his  first  conclu- 
sion, that  hydrogen  is  the  most  conspicuous  component 
of  the  prominences,  but  found  that  the  yellow  line  must 


198  THE   SUN. 

be  referred  to  some  other  element  than  sodium,  being 
somewhat  more  refrangible  than  the  D  lines. 

He  found  also  that,  by  slightly  moving  his  telescope 
and  causing  the  image  of  the  sun's  limb  to  take  different 
positions  with  reference  to  the  slit  of  his  spectroscope, 
he  could  even  trace  out  the  form  and  measure  the 
dimensions  of  the  prominences ;  and  he  remained  at 
his  station  for  several  days,  engaged  in  these  novel  and 
exceedingly  interesting  observations. 

Of  course,  he  immediately  sent  home  a  report  of  his 
eclipse- work,  and  of  his  new  discovery,  but,  as  his  sta- 
tion at  Guntoor,  in  eastern  India,  was  farther  from 
mail  communication  with  Europe  than  those  upon  the 
western  coast  of  the  peninsula,  his  letter  did  not  reach 
France  until  some  week  or  two  after  the  accounts  of 
the  other  observers;  when  it  did  arrive,  it  came  to 
Paris,  in  company  with  a  communication  from  Mr. 
Lockyer,  announcing  the  same  discovery,  made  inde- 
pendently, and  even  more  creditably,  since  with  Mr. 
Lockyer  it  was  not  suggested  by  anything  he  had  seen, 
but  was  thought  out  from  fundamental  principles. 

Nearly  two  years  previously  the  idea  had  occurred 
to  him  (and,  indeed,  to  others  also,  though  he  was  the 
first  to  publish  it)  that,  if  the  protuberances  are  gaseous, 
so  as  to  give  a  spectrum  of  bright  lines,  those  lines 
ought  to  be  visible  in  a  spectroscope  of  sufficient  power, 
even  in  broad  daylight.  The  principle  is  simply  this : 

Under  ordinary  circumstances  the  protuberances  are 
invisible,  for  the  same  reason  as  the  stars  in  the  day- 
time :  they  are  hidden  by  the  intense  light  reflected 
from  the  particles  of  our  own  atmosphere  near  the 
sun's  place  in  the  sky,  and,  if  we  could  only  sufficiently 
weaken  this  aerial  illumination,  without  at  the  same 
time  weakening  their  light,  the  end  would  be  gained. 


THE   CHROMOSPHERE  AND   THE  PROMINENCES.      199 

And  the  spectroscope  accomplishes  precisely  this  very 
thing.  Since  the  air-light  is  reflected  sunshine,  it  of 
course  presents  the  same  spectrum  as  sunlight,  a  con- 
tinuous band  of  color  crossed  by  dark  lines.  Now,  this 
sort  of  spectrum  is  greatly  weakened  by  every  increase 
of  dispersive  power,  because  the  light  is  spread  out  into 
a  longer  ribbon  and  made  to  cover  a  more  extended 
area.  On  the  other  hand,  a  spectrum  of  bright  lines 
undergoes  no  such  weakening  by  an  increase  in  the  dis- 
persive power  of  the  spectroscope.  The  bright  lines 
are  only  more  widely  separated — not  in  the  least  dif- 
fused or  shorn  of  their  brightness.  Moreover,  if  the 
gas  is  one  which,  like  hydrogen,  shows  dark  lines  in 
the  ordinary  solar  spectrum  (and  therefore  in  that  of 
the  air-light),  the  case  is  even  better :  not  only  is  the 
continuous  spectrum  of  the  air-light  weakened  by  the 
high  dispersion,  but  it  has  dark  gaps  in  it  just  where 
the  bright  lines  of  the  prominence  spectrum  will  fall. 

If,  then,  the  image  of  the  sun,  formed  by  a  telescope, 
be  examined  with  a  spectroscope,  one  might  hope  to 
see  at  the  edge  of  the  disk  the  bright  lines  belonging 
to  the  spectrum  of  the  prominences,  in  case  they  are 
really  gaseous. 

Mr.  Lockyer  and  Mr.  Huggins  both  tried  the  experi- 
ment as  early  as  1867,  but  without  success ;  partly  be- 
cause their  instruments  had  not  sufficient  power  to  bring 
out  the  lines  conspicuously,  but  more  because  they  did 
not  know  whereabouts  in  the  spectrum  to  look  for  them, 
and  were  not  even  sure  of  their  existence.  At  any  rate, 
as  soon  as  the  discovery  was  announced,  Mr.  Huggins 
immediately  saw  the  lines  without  difficulty,  with  the 
same  instrument  which  had  failed  to  show  them  to  him 
before.  It  is  a  fact,  too  often  forgotten,  that  to  per- 
ceive a  thing  known  to  exist  does  not  require  one  half 


200  THE 

the  instrumental  power  or  acuteness  of  sense  as  to  dis- 
cover it. 

Mr.  Lockyer,  immediately  after  his  suggestion  was 
published,  had  set  about  procuring  a  suitable  instrument, 
and  was  assisted  by  a  grant  from  the  treasury  of  the 
Eoyal  Society.  After  a  long  delay,  consequent  in  part 
upon  the  death  of  the  optician  who  had  first  under- 
taken its  construction,  and  partly  due  to  other  causes, 
he  received  the  new  spectroscope  just  as  the  report  of 
HerschePs  and  Tennant's  observations  reached  England. 
Hastily  adjusting  the  instrument,  not  yet  entirely  com- 
pleted, he  at  once  applied  it  to  his  telescope,  and  with- 
out difficulty  found  the  lines,  and  verified  their  position. 
He  immediately  also  discovered  them  to  be  visible 
around  the  whole  circumference  of  the  sun,  and  conse- 
quently that  the  protuberances  are  mere  extensions  of 
a  continuous  solar  envelope,  to  which,  as  mentioned 
above,  was  given  the  name  of  Chromosphere.  (He  does 
not  seem  to  have  been  aware  of  the  earlier  and  similar 
conclusions  of  Arago,  Grant,  Secchi,  and  others.)  He 
at  once  communicated  his  results  to  the  Royal  Society, 
and  also  to  the  French  Academy  of  Sciences,  and,  by 
one  of  the  curious  coincidences  which  so  frequently 
occur,  his  letter  and  Janssen's  were  read  at  the  same 
meeting,  and  within  a  few  minutes  of  each  other. 

The  discovery  excited  the  greatest  enthusiasm,  and 
in  1872  the  French  Government  struck  a  gold  medal 
in  honor  of  the  two  astronomers,  bearing  their  united 
effigies. 

It  immediately  occurred  to  several  observers,  Jans- 
sen,  Lockyer,  Zollner,  and  others,  that  by  giving  a  rapid 
motion  of  vibration  or  rotation  to  the  slit  of  the  spec- 
troscope it  would  be  possible  to  perceive  the  whole  con- 
tour and  detail  of  a  protuberance  at  once,  but  it  seems 


THE  CHROMOSPHERE  AND  THE  PROMINENCES.   201 

to  have  been  reserved  for  Mr.  Huggins  to  be  the  first 
to  show  practically  that  a  still  simpler  device  would 
answer  the  same  purpose.  With  a  spectroscope  of  suf- 
ficient dispersive  power  it  is  only  necessary  to  widen 
the  slit  of  the  instrument  by  the  proper  adjusting  screw. 
As  the  slit  is  widened,  more  and  more  of  the  protuber- 
ance becomes  visible,  and,  if  not  too  large,  the  whole 
can  be  seen  at  once :  with  the  widening  of  the  slit,  how- 
ever, the  brightness  of  the  background  increases,  so 
that  the  finer  details  of  the  object  are  less  clearly  seen, 
and  a  limit  is  soon  reached  beyond  which  further  widen- 
ing is  disadvantageous.  The  higher  the  dispersive  pow- 
er of  the  spectroscope  the  wider  the  slit  that  can  be 
used,  and  the  larger  the  protuberance  that  can  be  exam- 
ined as  a  whole — within  certain  limits,  Jiowever.  It  is 
not  difficult  with  our  latest  spectroscopes,  diffraction 
instruments  especially,  to  reach  a  dispersion  so  great 
that  even  the  C  line  becomes  broad  and  hazy,  like  the 

FIG.  46. 


Hu<JGiw8>  FIRST  OBSKRVATION  OF  A  PROMINENCE  IN  FULL  SUNSHINE. 

J>  lines  in  an  ordinary  instrument.  In  that  case  each 
luminous  point  in  the  prominence  itself  is  represented 
in  the  image  of  the  prominence,  net  by  a  point,  as  it 
should  be  to  give  clear  definition,  but  by  a  streak  at 
right  angles  to  the  spectrum  lines. 

Mr.  Huggins's  first   successful  observation  of  the 


202  THE  SUN, 

form  of  a  solar  protuberance  was  made  on  February  13, 
1869.  Fig.  46,  copied  from  the  "  Proceedings  of  the 
Koyal  Society,"  presents  his  delineation  of  what  he  saw. 
As  his  instrument  had  only  the  dispersive  power  of  two 
prisms,  and  included  in  its  field  of  view  a  large  portion 
of  the  spectrum  at  once,  he  found  it  necessary  to  sup- 
plement its  powers  by  using  a  red  glass  to  cut  off  stray 

FIG.  4T. 


SPECTROSCOPE,  WITH  TRAIN  OF  PRISMS. 

light  of  other  colors,  and  by  inserting  a  diaphragm  at 
the  focus  of  the  small  telescope  of  the  spectroscope  to 
limit  the  field  of  view  to  the  portion  of  the  spectrum 
immediately  adjoining  the  C  line.  With  the  instru- 
ments now  in  use,  these  precautions  are  seldom  neces- 
sary. 

It  may  be  noticed,  in  passing,  that  Mr.  Huggins  had 
previously  (and  has  subsequently)  made  many  experi- 
ments with  different  absorbing  media,  in  hopes  of  find- 
ing some  substance  which,  by  cutting  off  all  light  of 
other  color  than  that  emitted  by  the  prominences,  should 
render  them  visible  in  the  telescope  ;  thus  far,  however, 
without  success. 

The  spectroscopes  used  by  different  astronomers  for 


THE   CHROMOSPHERE   AND  THE  PROMINENCES.      203 

observations  of  this  sort  differ  greatly  in  form  and 
power.  Fig.  47  represents  the  one  long  used  at  the 
Shattuck  Observatory  of  Dartmouth  College,  and  sev- 
eral of  our  American  observatories  are  supplied  with 
instruments  similarly  arranged.  The  light  passes  from 
the  collimator  0,  through  the  train  of  prisms  p,  near 
their  bases,  and,  by  two  reflections  in  a  rectangular 
prism,  /*,  is  transferred  to  the  upper  story,  so  to  speak, 
of  the  prism-train,  and  made  to  return  to  the  telescope 
£,  finally  reaching  the  eye  at  e.  It  thus  twice  traverses 
a  train  of  six  prisms,  and  the  dispersive  power  of  the 
instrument  is  twelve  times  as  great  as  it  would  be  with 
only  one  prism.  The  diameter  of  the  collimator  is  a 
little  less  than  an  inch,  and  its  length  ten  inches.  The 
whole  instrument  only  weighs  about  fourteen  pounds, 
and  occupies  a  space  of  about  15  in.  X  6  in.  X  5  in. 
It  is  also  automatic,  i.  e.,  the  tangent  screw  m  keeps 
the  train  of  prisms  adjusted  to  their  position  of  min- 
imum deviation  by  the  same  movement  which  brings 
the  different  portions  of  the  spectrum  to  the  center  of 
the  field  of  view,  and  the  milled  head  f  focuses  both 
the  collimator  and  telescope  simultaneously. 

The  spectroscope  is  attached  to  the  equatorial  tele- 
scope, to  which  it  belongs,  by  means  of  the  clamping 
rings  #,  a.  These  slide  upon  a  stout  metal  rod,  firmly 
fastened  to  the  telescope  in  such  a  way  that  the  slit  s, 
of  the  instrument,  can  be  placed  exactly  at  the  focus  of 
the  object-glass,  where  the  image  of  the  sun  is  formed. 
This  instrument,  attached  to  the  telescope,  has  already 
b,een  figured  upon  page  74. 

Instruments  in  which  the  prism-train  is  replaced  by 
a  diffraction-grating  are  still  more  powerful ;  and  more 
convenient  also,  since  the  observer  has  the  great  advan- 
tage of  being  able  to  select,  within  certain  limits,  the 


204  THE  SUN. 

amount  of  dispersion  best  suited  to  his  purpose  by  sim- 
ply turning  the  grating  so  as  to  utilize  the  different 
orders  of  spectra  —  an  operation  easier  and  more  rapid 
than  that  of  rearranging  the  prism-train.  Diffraction 
spectroscopes  have,  however,  one  slight  disadvantage. 
When  used  with  the  open  slit,  the  forms  of  objects  seen 
through  the  slit  are  somewhat  distorted,  being  either 
compressed  or  extended  in  a  direction  at  right  angles 
to  the  slit.  When  the  grating  is  so  placed  that  the 
inclination  of  its  surface  to  the  view-telescope  is  greater 
than  to  the  collimator  (as  in  the  figure  on  page  68), 
compression  occurs.  In  this  case,  the  edge  of  the  slit 
being  placed  tangential  to  the  sun's  limb,  as  is  usual, 
prominences  on  the  edge  of  the  sun  appear  to  have  their 
height  reduced.  Of  course,  the  reverse  takes  place  when 
the  grating  is  placed  the  opposite  way.  This  distortion, 
however,  is  of  little  importance,  as  its  amount  is  easily 
calculated  and  allowed  for  when  necessary.*  A  similar 
distortion  is  produced  by  prismatic  spectroscopes  when 
the  prisms  are  not  adjusted  strictly  to  their  position 
of  minimum  deviation. 

The  diffraction  instruments,  which  the  writer  is  ac- 
customed to  use  for  solar  observations  at  Princeton, 
have  already  been  figured  on  pages  69  and  76. 

With  a  telescope  of  not  less  than  four  inches  aper- 
ture, equatorially  mounted,  and  a  spectroscope  of  dis- 
persive power  not  less  than  that  of  five  or  six  ordinary 
prisms,  the  observer  is  equipped  for  the  study  of  the 
chromosphere  and  prominences.  He  may  either  study 
the  spectrum  as  such,  using  the  instrument  with  a  nar- 


*  The  formula  for  the  calculation  is  simply  H  =  A        r»  in  which  H 

IB  the  true  height  of  the  object  seen  through  the  slit  ;  A  is  its  apparent 
height,  and  k  and  t  are  the  inclinations  of  the  surface  of  the  grating  to 
the  collimator  and  view-telescope  respectively. 


THE  CHROMOSPHERE  AND  THE  PROMINENCES.      205 

row  slit,  or  he  may  employ  it  with  widened  slit  simply 
as  a  means  of  viewing  the  prominences  and  studying 
their  forms  and  changes,  and,  so  far  as  this  goes — the 
study  of  forms  and  changes — very  little  is  gained  by 
large  telescopes  and  high  dispersion.  The  writer  has 
never  had  finer  views  of  prominences  than  those  fur- 
nished by  the  old  Dartmouth  telespectroscope.  When 
it  comes,  however,  to  the  study  of  minute  details,  espe- 
cially details  of  the  spectrum,  then  large  instruments 
are  indispensable. 

SPECTRUM   OF   THE   CHKOMOSPHERE   AND   PROMINENCES. 

The  spectra  of  the  chromosphere  and  prominences 
are  very  interesting  in  their  relations  to  that  of  the 
photosphere,  and  present  many  peculiarities  which  are 
not  yet  fully  explained.  At  times  and  in  places  where 
some  special  disturbance  is  going  on — frequently  in  the 
neighborhood  of  spots  at  the  times  when  they  are  just 
passing  around  the  limb  of  the  disk — the  spectrum,  at 
the  base  of  the  chromosphere,  is  very  complicated,  con- 
sisting of  hundreds  of  bright  lines.  In  the  course  of  a 
few  weeks  of  observation  at  Sherman  in  1872,  the  writer 
made  out  a  list  of  two  hundred  and  seventy-three,  and 
more  recent  observations  have  added  largely  to  the 
number — at  least  fifty  lines  within  the  limits  of  the 
visible  spectrum,  and,  by  photography,  at  least  eighty 
in  the  ultra-violet.  The  majority  of  the  lines,  how- 
ever, are  seen  only  occasionally,  for  a  few  minutes 
at  a  time,  when  the  gases  and  vapors,  which  generally 
lie  low,  mainly  in  the  interstices  of  the  clouds  which 
constitute  the  photosphere,  and  below  its  upper  surface, 
are  elevated  for  the  time  being  by  some  eruptive  action. 
For  the  most  part,  the  lines  which  appear  only  at  such 
times  are  simply  "reversals"  of  the  more  prominent 


206  THE  SUN. 

dark  lines  of  the  ordinary  solar  spectrum.  But  the  se- 
lection of  the  lines  seems  most  capricious  ;  one  is  taken > 
and  another  left,  though  belonging  to  the  same  element, 
of  equal  intensity,  and  close  beside  the  first.  It  is  evi- 
dent that  the  subject  needs  a  detailed  and  careful  study, 
combining  solar  observations  with  laboratory- work  upon 
the  spectra  of  the  elements  concerned,  before  a  satis- 
factory account  can  be  given  of  all  the  peculiar  behav- 
ior observed. 

The  lines  composing  the  true  chromosphere  spec- 
trum, if  we  may  call  it  so  (that  is,  those  which  are 
always  observable  in  it  with  suitable  appliances),  are 
not  very  numerous,  and  we  give  the  following  list, 
designating  them  by  their  wave-length,  as  given  by 
Rowland : 

1.  7065-50.  Helium. 

2.  6563-05,   C.  Hydrogen  (Ha). 

3.  5875-98,   D3.  (close  double).  Helium. 

4.  5316-87.   The  corona-line.  "  Coronium."    ? 

5.  4861-50,   F.  Hydrogen  (H/3). 

6.  4471-80,  /.  Helium. 

7.  4340-66,  g  (near  G).  Hydrogen  (H?). 

8.  4101-85,  h.  Hydrogen  (H8). 

9.  3970-20    (in  H).  Hydrogen  (He). 

10.  3968-56,   H.  Calcium. 

11.  3933-86,   K.  Calcium. 

The  first  line  is  generally  very  difficult  to  see,  though 
sometimes  pretty  conspicuous.  It  is  in  the  red,  between 
B  and  #,  and  has  a  very  faint  corresponding  dark  line. 
No.  3  has  no  dark  line  corresponding  as  a  usual  thing, 
though  occasionally  one  appears,  especially  in  the  neigh- 
borhood of  sun-spots.  No.  9  is  quite  within  the  broad 
shade  of  the  IE-line,  which  thus  appears  double  in  the 
chromosphere  spectrum. 

The  eleven  lines  mentioned  above  are  invariably 


THE   CHROMOSPHERE  AND   THE  PROMINENCES.      207 

present  in  the  spectrum  of  the  chromosphere ;  a  much 
larger  number  make  their  appearance  on  very  slight 
provocation.  They  are : 


1'.  6678-2. 

Helium. 

18'. 

4924-1. 

Iron. 

2'.  6431-1. 

Iron. 

19'. 

4922-3. 

Helium. 

3'.  6141-9. 

Barium. 

20'. 

4919-1. 

Iron.     ? 

4'.  5896-2,  D,. 

Sodium. 

21'. 

49003. 

Barium. 

5'.  5890-2,  Da. 

Sodium. 

22'. 

4584-1. 

Iron. 

6'.  5363-0. 

Iron.     ? 

23'. 

4501-4. 

Titanium. 

7'.  5284-6. 

Titanium  ?     ? 

24'. 

4491-5. 

Manganese. 

8'.  5276.2. 

Chromium.     ? 

25'. 

4490-2. 

Manganese. 

9'.  5234-7. 

Manganese. 

26'. 

4469-5. 

Iron. 

10'.  5198-2. 

?     ? 

27'. 

4245-5. 

Iron. 

11'.  5183-8,  fti. 

Magnesium. 

28'. 

4236-1. 

Iron. 

12'.  5172-9,  63. 

Magnesium. 

29'. 

4233-8. 

Iron. 

13'.  5169-2,  ba. 

Iron. 

SO'. 

4226-9. 

Calcium. 

14'.  5167-6,  64. 

Magnesium. 

31'. 

4215-7. 

Strontium. 

15'.  5018-6. 

Iron. 

32'. 

4077-9. 

Calcium. 

16'.  5015-8. 

Helium. 

33'. 

4026-0. 

Helium. 

17'.  4934-3. 

Barium. 

34'. 

3889-1. 

Hydrogen  (HO- 

It  is  not  intended,  however,  to  intimate  that,  if  one 
of  these  appears,  all  of  them  will  do  so,  nor  that  they 
are  equally  conspicuous  or  equally  common.  To  a  cer- 
tain degree,  also,  their  selection  by  the  writer  is  arbi- 
trary, for  there  are  nearly  as  many  more  which  are  seen 
pretty  frequently,  and  some  of  them  may  very  possibly 
be  found  hereafter  to  deserve  a  place  upon  the  list 
rather  than  some  that  have  been  included. 

It  requires  careful  manipulation  to  bring  out  the 
fainter  and  finer  lines  satisfactorily.  The  slit  must  be 
adjusted  with  extreme  care  to  the  focal  plane  of  the 
rays  under  examination,  placed  tangential  to  the  solar 
image,  and  brought  exactly  to  the  edge  of  the  disk.  A 
thousanth  of  an  inch  in  its  position  will  often  make  the 
whole  difference  between  a  successful  operation  and  its 
failure,  and  even  a  slight  unsteadiness  of  the  air  will 


THE  SUN. 


diminish  the  number  of  bright  lines  visible  by  at  least 
one  half. 

As  the  majority  of  the  lines  are  'developed  only  by 
more  or  less  unusual  disturbances  of  the  solar  surface, 
it  naturally  happens  that  one  very  often  finds  them  dis- 
torted or  displaced  by  the  motions  of  the  gases  along 
the  line  of  sight  (toward  or  from  the  observer),  as  ex- 
plained in  a  previous  chapter,  producing  what  Lockyer 
calls  "  motion-forms."  Occasionally,  also,  we  meet  with 
"  double  reversals,"  so  called,  especially  in  the  lines  of 
magnesium  and  sodium.  The  (dark)  lines  of  these  sub- 
stances are  rather  wide  in  the  solar  spectrum.  When 
reversed  in  the  chromosphere  spectrum,  the  phenome- 
non usually  consists  of  a  thin  bright  line  down  the  cen- 
ter of  the  wider  dark  band :  in  a  double  reversal  the 
bright  line  widens  and  a  fine  dark  line  appears  in  its 
center,  so  that  we  have  a  central  dark  line,  a  bright  one 
on  each  side  of  it,  and  outside  of  the  bright  lines  a  dark 
shade  on  both  sides.  Fig.  48  represents  such  a  double 

FIG.  48. 


DOTTBLE-BEVEBBAL  OP  THE  D-LlNEB.— (October,   1880.) 

reversal  of  the  D-lines  observed  by  the  writer  on  several 
occasions  in  1880.  The  phenomenon  seems  to  be  due 
to  the  presence  of  an  unusual  quantity  of  the  vapor  at  a 
considerable  density,  and  is  the  precise  correlative  of 


THE   CHROMOSPHERE  AND  TUB  PROMINENCES.      209 

what  is  sometimes  seen  in  the  spectrum  of  a  sodium- 
flame.  The  two  D-lines  of  sodium  each  becomes  itself 
double,  so  that  we  get  pairs  of  bright  lines  in  place  of 
single  lines.  The  electric  arc  often  shows  this  still 
more  finely. 

At  the  base  of  a  prominence,  the  C,  F,  H,  and  K 
lines  are  always  thus  doubly  reversed.     Fig.  49  is  from 

Fio.  49. 


DOUBLE-REVERSAL  OP  C-Lms. — (PHOTOGJRAPHED.) 

a  recent  photograph  of  the  C-line  obtained  at  Prince- 
ton, by  Mr.  Reed,  with  the  large  telescope  and  spectro- 
scope. The  slit  was  tangential  to  the  sun's  limb.  Of 
course,  an  isochromatic  plate  and  a  long  exposure  were 
required  to  get  such  an  impression  from  the  "ruby 
light"  of  that  part  of  the  spectrum.  When  the  slit  is 
adjusted  to  cross  the  sun's  limb  radially  the  bright 
lines  where  they  project  beyond  the  spectrum  of  the 
photosphere  assume  the  "arrow-headed"  form  shown 
in  Fig.  50. 

Generally  speaking,  the  spectrum  of  a  prominence 
15 


210 


THE   SUN. 


is  simpler  than  that  of  the  chromosphere  at  its  base. 
We  seldom  find  any  lines  except  C,  D,,  F,  g,  A,  H  and 


FIG.  50. 


K,  at  a  considerable  elevation  above  the  photosphere, 
though  f  is  sometimes  met  with.  On  rare  occasions, 
also,  the  vapors  of  sodium  and  magnesium  are  carried 
into  the  higher  regions,  and  once  or  twice  the  writer 
has  seen  the  line  No.  1  of  the  second  list  (6678*2)  in  the 
upper  portions  of  a  prominence. 

OBSERVATION    OF   PROMINENCES. 

When  the  spectroscope  is  used  as  a  means  of  ren- 
dering visible  the  forms  and  features  of  the  promi- 
nences, the  only  difference  is  that  the  slit  is  more  or  less 
widened. 

FIG.  51. 


OPINED  SLIT  OF  THE  SPECTBOSCOPB. 


The  telescope  is  directed  so  that  the  solar  image 
shall  fall  with  that  portion  of  its  limb  which  is  to  be 


THE   CHROMOSPHERE  AND   THE  PROMINENCES.      211 

examined  just  tangent  to  the  opened  slit,  as  in  Fig.  51, 
which  represents  the  slit-plate  of  the  spectroscope,  with 
the  image  of  the  sun  in  position  for  observation. 

If,  now,  a  prominence  exists  at  this  part  of  the  sun's 
limb  (as  would  probably  be  the  case,  considering  the 
proximity  of  the  spot  shown  in  the  figure),  and  if  the 
spectroscope  itself  is  so  adjusted  that  the  C-line  falls  in 
the  center  of  the  field  of  view,  then,  on  looking  into 
the  eye-piece,  one  will  see  something  much  like  Fig.  52. 
The  red  portion  of  the  spectrum  will  stretch  athwart 
the  field  of  view  like  a  scarlet  ribbon,  with  a  darkish 
band  across  it,  and  in  that  band  will  appear  the  promi- 
nences, like  scarlet  clouds — so  like  our  own  terrestrial 
clouds,  indeed,  in  form  and  texture,  that  the  resem- 
blance is  quite  startling  :  one  might  almost  think  he 
was  looking  out  through  a  partly-opened  door  upon  a 
sunset  sky,  except  that  there  is  no  variety  or  contrast  of 
color ;  all  the  cloudlets  are  of  the  same  pure  scarlet  hue. 
Along  the  edge  of  the  opening  is  seen  the  chromo- 
sphere, more  brilliant  than  the  clouds  which  rise  from 
it  or  float  above  it,  and  for  the  most  part  made  up  of 
minute  tongues  and  filaments.  Usually,  however,  the 
definition  of  the  chromosphere  is  less  distinct  than  that 
of  the  higher  clouds.  The  reason  is,  that  close  to  the 
limb  of  the  sun,  where  the  temperature  and  pressure 
are  highest,  the  hydrogen  is  in  such  a  state  that  the 
lines  of  its  spectrum  are  widened  and  "  winged,"  some- 
thing like  those  of  magnesium,  though  to  a  less  extent. 
Each  point  in  the  chromosphere,  therefore,  when  viewed 
through  the  opened  slit,  appears  not  as  a  point,  but  as 
a  short  lwe,  directed  lengthwise  in  the  spectrum.  As 
the  length  of  this  line  depends  upon  the  dispersive 
power  of  the  spectroscope,  it  is  easy  to  see  that  it  is 
possible  to  go  too  far  in  this  respect.  The  lower  the 


212  THE  SUN. 

dispersion  the  more  distinct  the  image  obtained,  but 
also  the  fainter  as  compared  with  the  background  upon 
which  it  is  seen. 

Just  beneath  the  chromosphere  (at  a  in  the  cut)  the 
appearance  is  as  if  the  edge  of  the  sun  was  dark,  a  phe- 
nomenon which  for  some  time  was  very  puzzling.  Its 
explanation  lies  in  the  "  double-reversal "  of  the  C-line 
at  the  base  of  the  chromosphere,  discussed  and  figured  a 
few  pages  back. 

FIG.  52. 


CHROMOSPHERE  AND  PROMINENCES  AS  SEEN  IN  THE  SPECTBUM. 

If  the  spectroscope  is  adjusted  upon  the  F-line,  in- 
stead of  C,  then  a  similar  image  of  the  prominences  and 
chromosphere  is  seen,  only  blue  instead  of  scarlet ;  usu- 
ally, however,  since  the  F-line  is  hazier  and  more  winged 
than  C,  this  blue  image  is  somewhat  less  perfect  in  its 
details  and  definition,  and  is  therefore  less  used  for 
observation.  Similar  effects  are  obtained  by  means  of 
the  yellow  line  near  D,  and  the  violet  line  near  G. 
With  suitable  precautions,  using  a  violet  shade-glass  be- 
fore the  eye,  and  carefully  shutting  out  all  extraneous 


THE  CHROMOSPHERE  AND  THE  PROMINENCES.      213 

light,  the  H  and  K  lines  can  also  be  used ;  but  visual 
observations  in  this  part  of  the  spectrum  are  extremely 
difficult  and  unsatisfactory. 

With  photography  the  case  is  the  reverse — these 
lines  are  then  precisely  those  which  can  be  employed 
most  easily  and  conveniently.  We  shall  recur  to  this  a 
little  later. 

Professor  Winlock  and  Mr.  Lockyer  have  attempted, 
by  using  an  annular  opening  instead  of  the  ordinary 
slit,  to  obtain  a  view  of  the  whole  circumference  of  the 
sun  at  once,  and  have  succeeded.  With  a  spectroscope 
of  sufficient  power,  and  adjustments  delicate  enough, 
the  thing  can  be  done ;  but  as  yet  no  very  satisfactory 
results  appear  to  have  been  reached.  We  still  (in  visual 
observations)  have  to  examine  the  circumference  piece- 
meal, so  to  speak,  readjusting  the  instrument  at  each 
point,  to  make  the  slit  tangential  to  the  limb. 

The  number  of  protuberances  of  considerable  mag- 
nitude (exceeding  ten  thousand  miles  in  altitude),  visible 
at  any  one  time  on  the  circumference  of  the  sun,  is 
never  very  great,  rarely  reaching  twenty-five  or  thirty. 
Their  number,  however,  varies  extremely  with  the  num- 
ber of  sun-spots :  during  a  sun-spot  minimum  there  are 
not  unfrequently  occasions  when  not  a  single  one  can 
be  found,  though  even  during  those  years  the  more 
usual  number  is  five  or  six — some  of  which  often  are 
of  considerable  size.  The  observations  of  Tacchini  and 
Secchi  have  showed  that  their  numbers  closely  follow 
the  march  of  the  sun-spots,  though  never  falling  quite 
so  low. 

To  Tacchini  we  owe  our  most  complete  record  of 
these  objects,  now  continuous  since  1872,  giving  their 
number  and  distribution  upon  the  sun,  with  drawings 
of  all  that  were  specially  remarkable.  Many  others 


214 


THE  SUN. 


have  co-operated  in  observations  of  this  kind  :  the  Hun- 
garian observers,  Fen yi  at  Kalocsa,  and  Yon  Gothard  at 
Hereny,  have  given  us  many  fine  descriptions  and  de- 
lineations. Father  Perry  and  his  assistant  Sidgreaves, 
at  Stonyhurst,  also  deserve  a  special  mention. 

Their  distribution  on  the  sun's  surface  is  in  some 
respects  similar  to  that  of  the  spots,  but  with  important 
differences.  The  spots  are  confined  within  40°  of  the 
sun's  equator,  being  most  numerous  at  a  solar  latitude 
of  about  20°  on  each  hemisphere.  Now,  the  protuber- 


RELATIVB  FREQUENCY  OF  PROTUBERANCES  AND  SUN-SPOTS. 

ances  are  most  numerous  precisely  where  the  spots  are 
most  abundant,  but  they  do  not  disappear  at  a  latitude 
of  40°  ;  they  are  found  even  at  the  poles,  and  from  the 
latitude  of  60°  actually  increase  in  number  to  a  latitude 
of  about  75°. 


THE   CHROMOSPHERE, AND  THE  PROMINENCES.      215 

The  annexed  diagram,  Fig.  53,  represents  the  rela- 
tive frequency  of  the  protuberances  and  spots  on  the 
different  portions  of  the  solar  surface.  On  the  left  side 
is  given  the  result  of  Carrington's  observation  of  1,386 
spots  between  1853  and  1861,  and  on  the  right  the  re- 
sult of  Secchi's  observations  of  2,767*  protuberances  in 
1871.  The  length  of  each  radial  line  represents  the 
number  of  spots  or  protuberances  observed  at  each  par- 
ticular latitude  on  a  scale  of  a  quarter  of  an  inch  to  the 
hundred ;  for  example,  Secchi  gives  228  protuberances 
as  the  number  observed  during  the  period  of  his  work 
between  10°  and  20°  of  south  latitude,  and  the  corre- 
sponding line  drawn  at  15°  south,  on  the  left-hand  side 
of  the  figure,  is  therefore  made  Jff  or  "57  of  an  inch 
long.  The  other  lines  are  laid  off  in  the  same  way,  and 
thus  the  irregular  curve  drawn  through  their  extremities 
represents  to  the  eye  the  relative  frequency  of  these 
phenomena  in  the  different  solar  latitudes.  The  dotted 
line  on  the  right-hand  side  represents  in  the  same  man- 
mer  and  on  the  same  scale  the  distribution  of  the  larger 
protuberances,  having  an  altitude  of  more  than  1',  or 
27,000  miles. 

A  mere  inspection  of  the  diagram  shows  at  once 
that,  while  the  prominences  may,  and  in  fact  often  do, 
have  a  close  connection  with  the  spots,  they  are  yet  to 
some  extent  independent  phenomena. 

A  careful  study  of  the  subject  shows  that  they  are 
much  more  closely  related  to  the  faculae.f  In  many 

*  The  2,767  prominences  are  not  all  different  ones.  If  any  of  the 
prominences  observed  on  one  day  remained  visible  the  next,  they  were 
recorded  afresh ;  and,  as  a  prominence  near  the  pole  would  be  carried 
but  slowly  out  of  sight  by  the  sun's  rotation,  it  is  thus  easy  to  see  how 
the  number  of  prominences  recorded  in  the  polar  regions  is  so  large. 

f  See  page  109. 


216  THE  SUN. 

cases  at  least,  faculse,  when  followed  to  the  limb  of  the 
sun,  have  been  found  to  be  surrounded  by  prominences, 
and  there  is  reason  to  suppose  that  the  fact  is  a  general 
one.  The  spots,  on  the  other  hand,  when  they  reach 
the  border  of  the  sun's  image,  are  commonly  surround- 
ed by  prominences  more  or  less  completely,  but  seldom 
overlaid  by  them.  Indeed,  Respighi  asserts  (and  the 
most  careful  observations  we  have  been  able  to  make 
confirm  his  statement)  that  as  a  general  rule  the  chro- 
mosphere is  considerably  depressed  immediately  over  a 
spot.  Secchi,  however,  denies  this. 

MAGNITUDE   AND    CLASSIFICATION   OF   PROMINENCES. 

The  protuberances  differ  greatly  in  magnitude.  The 
average  depth  of  the  chromosphere  is  not  far  from  10* 
or  12",  or  about  5,000  or  6,000  miles,  and  it  is  not,  there- 
fore, customary  to  note  as  a  prominence  any  cloud  with 
an  elevation  of  less  than  15"  or  20"— 7,000  to  9,000 
miles.  Of  the  2,767  already  quoted,  1,964  attained  an 
altitude  of  40",  or  18,000  miles,  and  it  is  worthy  of 
notice  that  the  smaller  ones  are  so  few,  only  about  one 
third  of  the  whole :  751,  or  nearly  one  fourth  of  the 
whole,  reached  a  height  of  over  1',  or  28,000  miles ;  the 
precise  number  which  reached  greater  elevations  is  not 
mentioned,  but  several  exceeded  3',  or  84,000  miles.  It 
is  only  rather  rarely  that  they  reach  elevations  as  great 
as  100,000  miles.  The  writer  has  in  all  seen,  perhaps, 
three  or  four  which  exceeded  150,000  miles,  and  Secchi 
has  recorded  one  of  300,000  miles.  On  October  7, 1880, 
the  writer  observed  one  which  attained  the  still  un- 
equaled  height  of  over  13'  of  arc,  or  350,000  miles. 
When  first  seen,  on  the  southeast  limb  ef  the  sun,  about 
10.30  A.  M.,  it  was  a  "  horn "  of  ordinary  appearance, 
some  40,000  miles  in  elevation,  and  attracted  no  special 


THE  CHROMOSPHERE  AND  THE  PROMINENCES.      217 

attention.  When  next  seen,  half  an  hour  later,  it  had 
become  very  brilliant  and  had  doubled  its  height :  dur- 
ing the  next  hour  it  stretched  upward  until  it  reached 
the  enormous  altitude  mentioned,  breaking  up  into  fila- 
ments which  gradually  faded  away,  until,  by  12.30  P.  M., 


ERUPTIVE  PROMINENCES. 


Three  figures,  of  the  same  prominence, 

seen  July  25.  1872. 

Fm.  64. 


Fio.  57. 


SPIKES. 


FIG.  58. 


SHEAF  AND  VOLUTES. 


FIG.  59. 


AS  BBKN  AT  8.30  P.  M. 

190,000  miles  to  the  inch. 


218 

there  was  nothing  left.  A  telescopic  examination  of  the 
sun's  disk  showed  nothing  to  account  for  such  an  ex- 
traordinary outburst,  except  some  small  and  not  very 
brilliant  faculse.  While  it  was  extending  upward  most 
rapidly  a  violent  cyclonic  motion  was  shown  by  the  dis- 
placement of  the  spectrum-lines,  and  H  and  K  were  re- 
versed through  its  whole  height. 

In  their  form  and  structure  the  protuberances  differ 
as  widely  as  in  their  magnitude.  Two  principal  classes 
are  recognized  by  all  observers — the  quiescent,  cloud- 
formed,  or  hydrogenous,  and  the  eruptive  or  metallic. 
By  Secchi  these  are  each  further  subdivided  into  several 
sub-classes  or  varieties,  between  which,  however,  it  is 
not  always  easy  to  maintain  the  distinctions. 

And  here  perhaps  is  the  proper  place  to  mention 
that  Trouvelot  insists  on  the  existence  of  "dark"  prom- 
inences— i.  e.,  clouds  of  cooler  hydrogen  that  absorb  the 
light  of  the  hydrogen  behind  them ;  but  there  is  no 
proof,  we  think,  that  these  are  anything  but  "holes." 
Tacchini,  on  the  other  hand,  is  disposed  to  assert  the 
existence  of  "  white  "  prominences,  which  give  a  con- 
tinuous spectrum,  and  so  are  not  reached  by  spectro- 
scopic  observation,  though  conspicuous  to  the  eye,  and 
on  the  photographic  plate,  at  the  time  of  a  total  eclipse, 
as  in  1883  and  December,  1889.  But  the  evidence  hardly 
warrants  confident  belief  in  the  existence  of  such  ob- 
jects. 

The  quiescent  prominences  in  form  and  texture  re- 
semble, with  almost  perfect  exactness,  our  terrestrial 
clouds,  and  differ  among  themselves  as  much  and  in  the 
same  manner.  The  familiar  cirrus  and  stratus  types  are 
very  common,  the  former  especially,  while  the  cumulus 
and  cumulo-stratus  are  less  frequent.  The  protuber- 
ances of  this  class  are  often  of  enormous  magnitude, 


THE   CHROMOSPHERE   AND  THE  PROMINENCES.      219 

• 
QUIESCENT     PROMINENCES. 

Scale,  75,000  miles  to  the  inch. 

FIG.  63. 


DIFFUSE. 


FIG.  61. 


FILAMENTARY. 


STEMMED. 


PLUMBS. 


FIG.  65. 


HOBNS. 


220  THE 

• 
especially  in  their  horizontal  extent  (but  the  highest 

elevations  are  attained  by  those  of  the  eruptive  order), 
and  are  comparatively  permanent,  remaining  often  for 
hours  and  days  without  serious  change ;  near  the  poles 
they  sometimes  persist  through  a  whole  solar  revolution 
of  twenty-seven  days.  Sometimes  they  appear  to  lie 
upon  the  limb  of  the  sun  like  a  bank  of  clouds  in  the 
horizon ;  probably  because  they  are  so  far  from  the 
edge  of  the  disk  that  only  their  upper  portions  are  in 
sight.  When  seen  in  their  full  extent  they  are  ordi- 
narily connected  to  the  underlying  chromosphere  by 
slender  columns,  which  are  usually  smallest  at  the  base, 
and  appear  often  to  be  made  up  of  separate  filaments 
closely  intertwined,  and  expanding  upward.  Some- 
times the  whole  under  surface  is  fringed  with  down- 
hanging  filaments,  which  remind  one  of  a  summer 
shower  falling  from  a  heavy  thunder-cloud.  Some- 
times they  float  entirely  free  from  the  chromosphere ; 
indeed,  as  a  general  rule,  the  layer  clouds  are  attended 
by  detached  cloudlets  for  the  most  part  horizontal  in 
their  arrangement. 

The  figures  give  an  idea  of  some  of  the  general  ap- 
pearances of  this  class  of  prominences,  but  their  delicate, 
filmy  beauty  can  be  adequately  rendered  only  by  a  far 
more  elaborate  style  of  engraving. 

Their  spectrum  is  usually  very  simple,  consisting  of 
the  four  lines  of  hydrogen,  and  the  three  of  helium, 
with  H  and  K.  Occasionally  the  sodium  and  mag- 
nesium lines  also  appear,  and  that  even  near  the  sum- 
mit of  the  clouds ;  and  this  phenomenon  was  so  much 
more  frequently  observed  in  the  clear  atmosphere  of 
Sherman  as  to  suggest  that,  if  the  power  of  our  spec- 
troscopes were  sufficiently  increased,  it  would  cease  to 
be  unusual. 


THE  CHROMOSPHERE  AND  THE  PROMINENCES.      221 

The  genesis  of  this  sort  of  prominence  is  problemat- 
ical. They  have  been  commonly  looked  upon  as  the 
debris  and  relics  of  eruptions,  consisting  of  gases  which 
have  been  ejected  from  beneath  the  solar  surface,  and 
then  abandoned  to  the  action  of  the  currents  of  the 
sun's  upper  atmosphere.  But  near  the  poles  of  the  sun 
distinctively  eruptive  prominences  never  appear,  and 
there  is  no  evidence  of  aerial  currents  which  would 
transport  to  those  regions  matters  ejected  nearer  the 
sun's  equator.  Indeed,  the  whole  appearance  of  these 
objects  indicates  that  they  originate  where  we  see  them. 
Possibly,  although  in  the  polar  regions  there  are  no 
violent  eruptions,  there  yet  may  be  a  quiet  outpouring 
of  heated  hydrogen  sufficient  to  account  for  their  pro- 
duction— an  outrush  issuing  through  the  smaller  pores 
of  the  solar  surface,  which  abound  near  the  poles  as 
well  as  elsewhere. 

But  Secchi  reports  an  observation  which,  if  correct, 
puts  a  very  different  face  upon  the  matter.*  He  has 
seen  isolated  cloudlets  form  and  grow  spontaneously 
without  any  perceptible  connection  with  the  chromo- 
sphere or  other  masses  of  hydrogen,  just  as  in  our  own 
atmosphere  clouds  form  from  aqueous  vapor,  already 

*  On  October  13,  1880,  the  writer  for  the  first  time  met  with  the 
same  phenomenon.  A  small,  bright  cloud  appeared  on  that  day,  about 
11  A.M.,  at  an  elevation  of  some  2-J-'  (67,500  miles)  above  the  limb,  with- 
out any  evident  cause  or  any  visible  connection  with  the  chromosphere 
below.  It  grew  rapidly  without  any  sensible  rising  or  falling,  and  in  an 
hour  developed  into  a  large  stratiform  cloud,  irregular  on  the  upper  sur- 
face, but  nearly  flat  beneath.  From  this  lower  surface  pendent  filaments 
grew  out,  and  by  the  middle  of  the  afternoon  the  object  had  become  one 
of  the  ordinary  stemmed  prominences,  much  like  Fig.  64. 

But  obviously  the  thing  is  very  unusual,  for  in  more  than  twenty 
years  of  observation  I  have  encountered  the  phenomenon  only  three 
times. 


222  THE  SUN- 

• 

present  in  the  air,  but  invisible  until  some  local  cooling 
or  change  of  pressure  causes  its  condensation.  These 
prominences  are,  therefore,  formed  by  some  local  heat- 
ing or  other  luminous  excitement  of  hydrogen  already 
present,  arid  not  by  any  transportation  and  aggregation 
of  materials  from  a  distance.  The  precise  nature  of 
the  action  which  produces  this  effect  it  would  not  be 
possible  to  assign  at  present ;  but  it  is  worthy  of  note 
that  the  spectroscopic  observations  made  during  eclipses 
rather  favor  this  view,  by  showing  that  hydrogen,  in  a 
feebly  luminous  condition,  is  found  all  around  the  sun, 
and  at  a  very  great  altitude — far  above  the  ordinary 
range  of  prominences. 

Indeed,  in  most  cases  the  forms  and  changes  of 
this  class  of  prominences  so  closely  resemble  our  own 
terrestrial  clouds  that  one  is  almost  forced  to  believe 
that  they  are  surrounded  by,  and  float  in,  a  medium 
which  does  not  greatly  differ  from  themselves  in  den- 
sity, though  it  is  not  visible  in  the  spectroscopic  mode 
of  observation. 

ERUPTIVE    PROMINENCES. 

The  eruptive  prominences  are  very  different — much 
more  brilliant  and  much  more  vivacious  and  interesting. 
They  consist  usually  of  brilliant  spikes  or  jets,  which 
change  their  form  and  brightness  very  rapidly.  For  the 
most  part  they  attain  altitudes  of  not  more  than  20,000 
or  30,000  miles,  but  occasionally  they  rise  far  higher  than 
even  the  largest  of  the  clouds  of  the  preceding  class. 
Their  spectrum  is  very  complicated,  especially  near 
their  base,  and  often  filled  with  bright  lines,  those  of 
sodium,  magnesium,  barium,  iron,  and  titanium,  being 
especially  conspicuous,  while  calcium,  chromium  man- 
ganese, and  probably  sulphur,  are  by  no  means  rare, 


THE  CHROMOSPHERE  AND  THE  PROMINENCES.      223 

Scale,  T5,000  miles  to  the  inch. 


PIG.  66. 


FIG.  69. 


PROMINENCE  AB  IT  APPEARED  AT  HALF-PAS* 
TWELVE  O'CLOCK,  SEPTEMBER  7,  18T1. 

FIG.  70. 


VERTICAL  FILAMENTS. 
FIG.  67. 


AS  THE  ABOVE  APPEARED  HALF  AN  HOUR  LATER, 
WHEN  THE  UP-RUSHING  HYDROGEN  ATTAINED  A 
HEIGHT  OF  MORE  THAN  200,000  MILES. 

FIG.  71. 


FLAMES. 


SPOT   NEAR  THB  SlTW'B  LlMB,   WITH   ACCOMPAJfYIW* 

JETS  OF  HYDBOGBN,  AS  SEKN  OCTOBER  ft.  1871. 


224:  THE  SUN. 

and  for  this  reason  Secchi  calls  them  metallic  promi- 
nences. 

They  usually  appear  in  the  immediate  neighborhood 
of  a  spot,  never  occurring  very  near  the  solar  poles. 
Their  form  and  appearance  change  with  great  rapidity, 
so  that  the  motion  can  almost  be  seen  with  the  eye — an 
interval  of  fifteen  or  twenty  minutes  being  often  suffi- 
cient to  transform,  quite  beyond  recognition,  a  mass  of 
these  flames  fifty  thousand  miles  high,  and  sometimes 
embracing  the  whole  period  of  their  complete  develop- 
ment or  disappearance.  Sometimes  they  consist  of 
pointed  rays,  diverging  in  all  directions,  like  hedgehog- 
spines.  Sometimes  they  look  like  flames ;  sometimes 
like  sheaves  of  grain ;  sometimes  like  whirling  water- 
spouts, capped  with  a  great  cloud;  occasionally  they 
present  most  exactly  the  appearance  of  jets  of  liquid 
fire,  rising  and  falling  in  graceful  parabolas ;  frequently 
they  carry  on  their  edges  spirals  like  the  volutes  of  an 
Ionic  column;  and  continually  they  detach  filaments 
which  rise  to  a  great  elevation,  gradually  expanding 
and  growing  fainter  as  they  ascend,  until  the  eye  loses 
them.  Our  figures  present  some  of  the  more  common 
and  typical  forms,  and  illustrate  their  rapidity  of  change, 
but  there  is  no  end  to  the  number  of  curious  and  inter- 
esting appearances  which  they  exhibit  under  varying 
circumstances. 

The  velocity  of  the  motions  often  exceeds  a  hundred 
miles  a  second,  and  sometimes,  though  very  rarely, 
reaches  two  hundred  miles.  That  we  have  to  do  with 
actual  motions,  and  not  with  mere  change  of  place  of  a 
luminous  form,  is  rendered  certain  by  the  fact  that  the 
lines  of  the  spectrum  are  often  displaced  and  distorted 
in  a  manner  to  indicate  that  some  of  the  cloud-masses 
are  moving  either  toward  or  from  the  earth  (and,  of 


THE  CHROMOSPHERE  AND  THE  PROMINENCES.      225 

course,  tangential   to  the   solar .  surface)  with   similar 
swiftness. 

Fig.  72  is  a  representation  of  a  portion  of  the  spec- 
trum of  a  prominence  observed  at  Sherman  on  Angust 
3,  1872,  an  observation  to  which  allusion  was  made  in 


FIG.  72. 


F  LINE;    SPECTRUM  or  CHROMOSPHERE, Aue.3?-°  1872. 


the  preceding  chapter.  The  F-line,  at  208  of  the  scale, 
must  be  imagined  as  blazingly  brilliant,  and  fainter 
bright  lines  appear  at  203-2,  208-8,  209*4,  and  212-1  (the 
scale  is  Kirchhoff  Js),  while  two  bands  of  continuous  spec- 
trum, produced  probably  by  the  compression  of  the  gas 
at  the  points  of  maximum  disturbance,  run  the  whole 
length  of  the  figure.  At  the  upper  point  of  disturbance 
F  is  drawn  out  into  a  point  reaching  to  207*4  of  the 
scale,  and  indicating  a  velocity  of  230  miles  a  second 
away  from  us ;  at  the  lower  point  it  extends  to  208*7, 
and  indicates  a  velocity  of  about  250  miles  per  second 
toward  us.  It  was  very  noticeable  that  this  swift 
motion  of  the  hydrogen  did  not  seem  to  carry  with  it 

many  other  substances  which  were  at  the  time  repre- 
16 


226  THE   SUN. 

sented  in  the  spectrum  by  their  bright  lines;  mag- 
nesium and  sodium  were  somewhat  affected,  but  barium 
and  the  unknown  element  of  the  corona  were  not. 

When  we  inquire  what  forces  impart  such  a  velocity, 
the  subject  becomes  difficult.  If  we  could  admit  that 
the  surface  of  the  sun  is  solid,  or  even  liquid,  as  Zollner 
thinks,  then  it  would  be  easy  to  understand  the  phe- 
nomena as  eruptions,  analogous  to  those  of  volcanoes 
on  the  earth,  though  on  the  solar  scale.  But.  it  is  next 
to  certain  that  the  sun  is  mainly  gaseous,  and  that  its 
luminous  surface  or  photosphere  is  a  sheet  of  incandes- 
cent clouds,  like  those  of  the  earth,  except  that  water- 
droplets  are  replaced  by  droplets  of  the  metals ;  and  it 
is  difficult  to  see  how  such  a  shell  could  exert  sufficient 
confining  power  upon  the  imprisoned  gases  to  explain 
such  tremendous  velocity  in  the  ejected  matter. 

Possibly  the  difficulty  may  be  met  by  taking  account 
of  the  enormous  amount  of  condensation  which  must  be 
going  on  within  the  photosphere.  To  supply  the  heat 
which  the  sun  throws  off  (enough  to  melt  each  minute 
a  shell  of  ice  nearly  fifty  feet  thick  over  his  entire  sur- 
face) would  require  the  condensation  of  enough  vapor 
to  make  a  sheet  of  liquid  six  feet  thick  in  the  same  time 
— supposing,  that  is,  the  latent  heat  of  the  solar  vapors 
not  greater  than  that  of  water  vapors.  This,  of  course, 
is  uncertain,  but,  so  far  as  we  know,  very  few  if  any 
vapors  contain  more  latent  heat  than  that  of  water,  and 
we  may  therefore  consider  it  roughly  correct  to  estimate 
the  continuous  production  of  liquid  as  measured  by  the 
quantity  named.  Now,  on  the  surface  of  the  earth  a 
rain-storm  which  deposits  two  inches  in  an  hour  is  very 
uncommon — in  such  a  storm  the  water  falls  in  sheets. 
If  we  admit,  then,  that  any  considerable  portion  of  the 
sun's  heat  is  due  to  such  a  condensation  of  the  solar 


THE  CHROMOSPHERE  AND  THE  PROMINENCES.      22Y 

vapors,  it  is  easy  to  see  that  the  quantity  of  liquid  pour- 
ing from  the  solar  clouds  must  be  so  enormous  that  the 
drops  could  not  be  expected  to  remain  separate,  but 
will  almost  certainly  unite  into  more  or  less  continuous 
masses  or  sheets,  between  and  through  which  the  gases 
ascending  from  beneath  must  make  their  way.  And, 
since  the  weight  of  the  vapors  which  ascend  must  con- 
tinually equal  that  of  the  products  of  condensation 
which  are  falling,  it  is  further  evident  that  the  upward 
currents,  rushing  through  contracted  channels,  must 
move  with  enormous  velocity,  and  therefore,  of  course, 
that  the  pressure  and  temperature  must  rapidly  increase 
from  the  free  surface  downward.  It  would  seem  that 
thus  we  might  explain  how  the  upper  surface  of  the 
hydrogen  atmosphere  is  tormented  by  the  up-rush  from 
below,  and  how  gaseous  masses,  thrown  up  from  be- 
neath, should,  in  the  prominences,  present  the  appear- 
ances which  have  been  described.  Nor  would  it  be 
strange  if  veritable  explosions  should  occur  in  the  quasi 
pipes  or  channels  through  which  the  vapors  rise,  when, 
under  the  varying  circumstances  of  pressure  and  tem- 
perature, the  mingled  gases  reach  their  point  of  combi- 
nation ;  explosions  which  would  fairly  account  for  such 
phenomena  as  those  represented  in  Figs.  69  and  70,  when 
clouds  of  hydrogen  were  thrown  to  an  elevation  of  more 
than  200,000  miles  with  a  velocity  which  must  have 
exceeded  at  first  200  miles  per  second,  and  very  prob- 
ably, taking  into  account  the  resistance  of  the  solar 
atmosphere,  may,  as  Mr.  Proctor  has  shown,  have  ex- 
ceeded 500 ;  a  velocity  sufficient  to  hurl  a  dense  material 
entirely  clear  of  the  power  of  the  sun's  attraction,  and 
send  it  out  into  space,  never  to  return. 

And  yet  such  velocities  so  far  exceed   those  with 
which  we  are  familiar  here  that  it  is  not  strange  at  all 


228  THE  SUN. 

that  there  should  be  reluctance  to  admit  them,  and  at- 
tempts  to  substitute  for  such  motions  of  material  masses 
the  motion  of  mere  forms  and  the  swift  transference 
of  regions  of  luminosity  through  gases  themselves  at 
rest — just  as  when  a  flash  runs  from  one  end  to  the 
other  of  a  long  train  of  gunpowder,  or  a  suddenly 
kindled  flame  flies  up  through  a  chimney.  In  many 
respects  such  conceptions  perfectly  represent  the  facts 
— prominences  apparently  at  rest  might  be  like  water- 
falls or  gas-flames — mere  stationary  forms  made  up 
from  a  steady  succession  of  material  particles ;  and  the 
swiftly  moving  ones,  so  far  as  appearances  go,  might  be 
flashes  traveling  swiftly  through  extensive  masses  of  gas 
comparatively  motionless.  If  such  a  view  is  tenable,  then 
we  might  imagine,  as.  Brester  has  done,  that  the  sun  is 
quiescent  and  serene,  composed  of  overlying  strata  of 
different  density,  each  in  a  state  of  stable  equilibrium, 
such  that  any  considerable  vertical  motions  are  impossi- 
ble, and  horizontal  disturbances  soon  checked :  what 
look  to  us  like  fiery  flames  and  furious  commotions  are 
then  only  like  the  auroral  flickerings  in  our  own  atmos- 
phere. 

But  to  this  view  the  one  conclusive  objection,  unless 
it  can  be  evaded,  is  the  fact  that  the  lines  in  the  spec- 
trum testify  to  swift  motions  in  the  line  of  vision — that 
masses  of  hydrogen  and  helium,  of  iron-vapor  and  cal- 
cium, are  shown  to  be  moving  toward  or  from  us  with  tre- 
mendous velocity.  Brester  therefore  maintains,  and  oth- 
ers with  him,  that  though  unquestionably  the  motion  of 
a  luminous  mass  of  matter  toward  or  from  the  observer 
will  produce  such  line-displacements  as  are  observed,  we 
are  not  shut  up  to  that  as  their  only  explanation.  He 
maintains  that  the  motion  of  a  mere  luminous  form 
would  produce  the  same  effect :  that  if  a  train  of  pow- 


THE  CHROMOSPHERE  AND   THE  PROMINENCES.      229 

der,  for  instance,  were  laid  straight  away  from  us  for 
a  distance  of  ten  miles,  then  if  it  were  lighted  at  the  end 
nearest  us  and  the  flash  reached  the  other  end  in  ten 
seconds,  the  spectrum  of  the  traveling  flash  would  indi- 
cate a  receding  velocity  of  one  mile  a  second.  There 
is,  however,  no  evidence  to  support  such  a  doctrine. 
No  theoretical  reason  can  be  assigned — at  least  none  has 
been  so  far  as  we  know — why  the  phases  of  the  light- 
waves issuing  from  the  flash  at  each  point  in  its  ten- 
mile  course  should  reach  the  observer  with  the  same 
regular  retardation  as  in  the  case  of  a  luminous  ball 
moving  over  the  same  path  with  the  same  speed :  and 
unless  this  condition  is  observed,  or  something  essen- 
tially equivalent,  Doppler's  principle  has  no  application. 
As  for  experimental  evidence,  none  exists  as  yet,  nor 
do  we  know  of  any  proposed  method  by  which  the  hy- 
pothesis can  be  tested. 

Still  a  different  and  very  curious  theory  of  the  solar 
constitution  has  been  lately  proposed  by  Schmidt,  of 
Stuttgart,  and  a  good  deal  discussed  (rather  favorably, 
too,  on  account  of  its  mathematical  interest)  by  various 
writers.  It  amounts  to  this :  that  the  sun  is  a  great 
globe  of  heated  transparent  gas,  much  denser  in  the 
center,  and  the  apparent  definiteness  of  outline  is  due 
to  the  curious  refraction  of  the  light  in  such  a  medium. 
The  rays  from  points  behind  the  sun's  center,  according 
to  this  theory,  reach  us  from  all  around  the  limb ;  pho- 
tosphere and  chromosphere  are  an  optical  jumble  of 
rays  from  widely  different  points  within  the  globe,  and 
most  of  the  phenomena  we  see  on  and  about  the  solar 
surface  are  purely  optical,  like  halos,  rainbows,  and 
mirages. 

It  is  probably  sufficient  to  point  out  that  a  gaseous 
globe  which  contains  in  itself  quantities  of  metallic  vapor 


230  THE 

can  not  remain  wholly  gaseous  for  any  length  of  time. 
In  its  outer  regions,  where  it  is  exposed  to  the  cold  of 
space,  condensation  must  inevitably  take  place,  incan- 
descent clouds  must  gather,  and  a  "  photosphere  "  must 
form — it  must  "  clothe  itself  with  light  as  with  a  gar- 
ment." The  theory  can  apply  only  to  a  mass  composed 
wholly  of  "  permanent "  gases — those  that  will  not  turn 
to  liquid  or  solid  even  at  the  lowest  temperatures  to 
which  they  are  anywhere  exposed.  It  may  be  that  in 
the  planetary  nebulae  we  have  such  bodies. 

PROMINENCE    PHOTOGRAPHY. 

As  far  back  as  1870  attempts  were  made  by  the 
writer  to  photograph  the  prominences,  and  a  partial 
success  was  reached.  A  little  camera  carrying  a  sensi- 
tized microscope  slide  was  fitted  to  the  spectroscope 
figured  on  page  202,  and  with  a  four-minute  exposure  a 
distinct  impression  of  a  prominence  was  obtained.  The 
hydrogen  line  employed  was  g,  (Hy).  It  was  in  the  days 
of  the  wet-plate  collodion  process,  and  the  necessity  of 
so  long  an  exposure  made  it  certain  that  it  would  not 
be  worth  while  then  to  follow  up  the  matter.  But  the 
introduction  of  the  modern  dry-plate  has  changed  all 
that.  The  subject  was  resumed  in  1889  and  1890  almost 
simultaneously  by  Deslandres,  in  France,  and  George 
E.  Hale,  in  Chicago.  By  1891  it  had  become  possible 
to  produce  very  fair  pictures  of  moderate-sized  prom- 
inences by  using  the  H  or  K  lines  with  a  powerful 
spectroscope,  and  putting  a  photographic  plate  in  place 
of  the  eye.  If  the  slit  is  narrow  we  get  merely  the 
double  reversal  of  H  and  K,  as  shown  in  Fig.  73.  (It 
is  worth  while  to  notice,  in  passing,  the  hydrogen  line 
(He)  which  is  so  near  to  H.  For  years  it  had  been  a 
puzzle  why  in  the  spectra  of  stars  of  the  so-called  "  first " 


THE  CHROMOSPHERE 'AND 'THE  PROMINENCES.      231 

class,  like  Vega,  H  should  be  conspicuous  and  K  miss- 
ing. The  discovery  of  this  hydrogen  line  by  Ames 
solves  the  problem.  In  Yega's  spectrum  H  is  the  hydro- 
gen, not  the  calcium  band.)  To  return  :  if  now  we  sim- 
ply open  the  slit  as  far  as  can  safely  be  done,  we  get  the 

Fro.  73. 


DOUBLE-REVERSAL  OF  H  AND  K  LINES. 

image  of  the  prominence  in  each  of  the  two  bands,  as 
in  Fig.  74.  An  exposure  of  five  seconds  is  abundant. 
Indeed,  at  Princeton,  by  using  isochromatic  plates  with 
an  exposure  of  several  minutes,  we  have  been  able  to 
photograph  prominences  even  in  the  C-line  (Fig.  75). 
But  this  requires  extremely  accurate  adjustment  of  the 
clockwork  of  the  telescope  and  careful  manipulation. 

With  this  open-slit  arrangement,  however,  we  are 
limited  to  prominences  that  are  not  very  large ;  nor  is 
the  definition  very  perfect. 

These  difficulties  may  be  avoided  by  adopting  an 
arrangement  long  ago  suggested  by  Janssen  and  others 


232 


THE  SUN. 


in  the  early  days  of  prominence   observation.      The 
spectroscope  is  fitted  up  with  a  second  slit  at  the  eye 


Fio.  74. 


PROMINENCE  IN  H  AND  K  LINKS. — (FROM  PHOTOGRAPH.) 

end  of  the  view-telescope,  and  in  some  forms  of  the  ap- 
paratus both  the  collimator  slit  and  the  other  are  free 
to  slide  back  and  forth  in  the  focal  plane  and  length- 
Em.  75. 


PROMINENCE  PHOTOGRAPHED  IN  C-LiNE.— REED. 

wise  of  the  spectrum.     Beginning  with  the  slits  each  at 
the  center  of  its  slide,  suppose  the  prism  or  grating  to 


THE   CHROMOSPHERE  AND   THE  PROMINENCES.      233 

be  so  adjusted  as  to  bring  the  K  line  into  position  to  be 
seen  through  the  second  slit ;  if  now  we  slide  the  colli- 
mator  slit,  the  K  line  will  move  away  from  the  second 
slit,  and  to  keep  it  in  view  this  one  will  have  to  slide 
also.  This  can  be  automatically  affected :  the  two 
sliders  that  carry  the  slits  can  be  connected  mechanic- 

Fio.  76. 


BALE'S  SPECTRO-HELIOGBAPH. 

ally  in  various  ways  so  that  their  motions  shall  exactly 
correspond ;  and  if  we  add  a  photographic  plate  and  its 
accessories  we  have  the  so-called  "  spectro-heliograph." 
Fig.  76  is  from  a  photograph  of  Professor  Hale's 
instrument,  as  used  in  1892  at  the  Kenwood  Astro- 
Physical  Observatory,  in  Chicago. 


334  THE 

In  order  to  photograph  a  prominence  the  telescope 
is  so  directed  as  to  bring  the  base  of  the  prominence  to 
the  collimator  slit — the  slit  being  tangential  to  the  sun's 
limb.  The  clockwork  of  the  equatorial  will  keep  it 
there  if  properly  adjusted.  Then  the  collimator  slit  is 
made  to  slide  smoothly  and  gradually  upward  to  the 
top  of  the  prominence  (by  a  hydraulic  apparatus  in 
Hale's  instrument),  and  at  the  same  time  the  other  slit 
travels  in  front  of  the  sensitive  plate,  so  that  this  re, 
ceives,  one  after  the  other,  the  imprints  of  all  the  suc- 
cessive sections  of  the  prominence.  As  an  example  of 
a  prominence  photograph  so  made,  we  give  Fig.  77, 
which  was  taken  March  25, 1895.  The  maximum  eleva- 
tion was  very  nearly  281,000  miles.  The  vertical  dark 
streaks  are  "  dust-lines  "  caused  by  motes  in  the  slit  or 
roughness  of  its  edges ;  the  streaks  at  right  angles  to 

FIG.  77. 


PROMINENCE  or  MARCH  25,  1895. — SPECTBO-HBLIOQBAPH  PHOTOGRAPH. 

these  are  due  to  slight  unsteadiness  in  the  sliding  mo- 
tion, produced  by  the  hydraulic  "  clepsydra." 

If  the  image  of  the  sun  itself  is  covered  by  an 
opaque  disk  of  exactly  the  right  size,  then  the  slits  may 


THE   CHROMOSPHERE   AND   THE  PROMINENCES.      235 

be  made  to  traverse  the  whole  chromosphere,  rather 
slowly,  at  a  single  journey,  and  we  shall  get  at  one  ex- 
posure a  picture  of  the  whole  array  of  prominences 


FIG.  78. 


SPECTRO-HELIOGRAPH  PHOTOGRAPH  OF  THE  ENTIRE  CHROMOSPHERE. 

surrounding  the  sun  at  that  time.  Figs.  78  and  79  were 
made  in  this  way  at  Kenwood  Observatory,  though  we 
can  not  give  the  exact  date. 

If  after  making  such  an  exposure  the  screen  that 
covered  the  sun's  disk  is  removed,  and  the  collimator 
slit  is  made  to  retrace  its  path  (swiftly  this  time),  we 
shall  get,  not  only  the  chromospheric  ring  with  its  out- 
lying prominences,  but  the  whole  surface  of  the  sun 
itself  as  seen  by  the  monchromatic,  "K-line,"  light. 
The  faculous  regions  come  out  with  special  emphasis. 
The  reader  will  recall  the  pending  discussion  with  re- 
gard to  their  real  nature  and  elevation,  referred  to  on 
page  109.  Fig.  31*  on  that  page  is  an  example  of  this 
sort — also  from  Mr.  Hale. 

The  "  two-slit "  arrangement  admits  of  various  mod- 
ifications :  in  one  the  spectroscope  and  its  slits  are  fixed, 
the  image  of  the  sun  is  allowed  to  drift  over  the  colli- 


236  THE   SUN. 

mator  slit  by  the  diurnal  motion,  and  the  photographic 
plate  is  drawn  along  at  the  same  rate,  and  in  the  same 
direction,  by  a  suitably  adjusted  clockwork.  In  this 
case  the  large  telescope  that  forms  the  sun's  image  is 
usually  also  fixed  in  a  horizontal  position,  and  the  sun's 
rays  are  directed  into  it  by  a  plane  mirror,  as  in  the 
American  transit  of  Venus  apparatus.  The  instrument 
of  Deslandres  is  arranged  in  this  manner,  and  with  it  he 
has  obtained  all  the  results  that  Hale  has  reached. 

Fie.  79. 


SPECTEO-HELIOOKAPH  PHOTOGRAPH  OF  PROMINENCES. 

In  the  great  spectro-heliograph  now  building  under 
Mr.  Hale's  direction,  to  be  used  with  the  gigantic 
forty-inch  equatorial  of  the  Yerkes  Observatory,  the 
slits  will  be  fixed  in  the  spectroscope,  but  the  whole  spec- 
troscope will  be  so  arranged  in  the  framework  which 
attaches  it  to  the  equatorial  that  it  can  be  moved  bodily 
across  the  seven-inch  image  of  the  sun,  while  the  plate- 
holder  remains  fixed. 

Students  of  solar  physics  await  with  great  interest 
the  outcome  of  the  new  methods  and  apparatus. 


CHAPTEE  YIL 

THE  CORONA. 

General  Appearance  of  the  Phenomenon. — Various  Representations. — 
Eclipses  of  1857,  1860,  1867,  1868,  1869,  1871,  1878,  1882,  1889, 
and  1893. — Proof  that  the  Corona  is  mainly  a  Solar  Phenomenon. — 
Brightness  of  the  Corona. — Connection  with  Sun-Spot  Period. — Spec- 
trum of  the  Corona. — Application  of  the  Analyzing  and  Integrating 
Spectroscopes. — Polarization. — Evidence  of  the  Slitless  Spectroscope 
as  to  the  Constitution  of  the  Corona. — Changes  and  Motions  in  the 
Corona. — Its  Form  and  Constitution,  and  Theories  as  to  its  Nature 
and  Origin. 

A  TOTAL  eclipse  of  the  sun  is  unquestionably  one  of 
the  most  impressive  of  all  natural  phenomena,  and  the 
corona,  or  aureole  of  light,  which  then  surrounds  the 
sun,  is  its  most  impressive  feature.  On  such  an  occa- 
sion, if  the  sky  is  clear,  the  moon  appears  of  almost 
inky  darkness,  with  just  sufficient  illumination  at  the 
edge  of  the  disk  to  bring  out  its  rotundity  in  a  strik- 
ing manner.  It  looks  not  like  a  flat  screen,  but  like 
a  huge  black  ball,  as  it  really  is.  From  behind  it 
stream  out  on  all  sides  radiant  filaments,  beams,  and 
sheets  of  pearly  light,  which  reach  to  a  distance  some- 
times of  several  degrees  from  the  solar  surface,  forming 
an  irregular  stellate  halo,  with  the  black  globe  of  the 
moon  in  its  apparent  center.  The  portion  nearest  the 
sun  is  of  dazzling  brightness,  but  still  less  brilliant  than 
the  prominences,  which  blaze  through  it  like  carbun- 
cles. Generally  this  inner  corona  has  a  pretty  uni- 
form height,  forming  a  ring  three  or  four  minutes  of 


238 

arc  in  width,  separated  by  a  somewhat  definite  outline 
from  the  outer  corona,  which  reaches  to  a  much  greater 
distance,  and  is  far  more  irregular  in  form.  Usually 
there  are  several  "  rifts,"  as  they  have  been  called,  like 
narrow  beams  of  darkness,  extending  from  the  very 
edge  of  the  sun  to  the  outer  night,  and  much  resem- 
bling the  cloud-shadows  which  radiate  from  the  sun 
before  a  thunder-shower.  But  the  edges  of  these  rifts 
are  frequently  curved,  showing  them  to  be  something 
else  than  real  shadows.  Sometimes  there  are  narrow, 
bright  streamers,  as  long  as  the  rifts,  or  longer.  These 
are  often  inclined,  occasionally  are  even  nearly  tangen- 
tial to  the  solar  surface,  and  frequently  are  curved.  On 
the  whole,  the  corona  is  usually  less  extensive  and  brill- 
iant over  the  solar  poles,  and  there  is  a  recognizable 
tendency  to  accumulations  above  the  middle  latitudes, 
or  spot-zones ;  so  that,  speaking  roughly,  the  corona 
shows  a  disposition  to  assume  the  form  of  a  quadrilat- 
eral or  four-rayed  star,  though  in  almost  every  individual 
case  this  form  is  greatly  modified  by  abnormal  streamers 
at  some  point  or  other. 

Unlike  the  chromosphere,  which  seems  first  to  have 
been  observed,  as  was  mentioned  in  the  previous  chap- 
ter, only  a  little  more  than  a  century  ago,  the  corona 
has  been  known  from  antiquity,  and  is  described  by 
Philostratus  and  Plutarch  in  almost  the  same  terms  we 
should  ourselves  employ.  And  yet  our  knowledge  of 
it  remains  very  limited.  The  chromosphere  and  promi- 
nences we  can  now  reach  and  study,  comparatively  at 
our  leisure,  by  the  help  of  the  spectroscope;  but  the 
corona  is  still  inaccessible,  except  during  the  short  and 
precious  moments  of  a  total  eclipse — in  all,  not  more 
than  a  few  days  in  a  century — so  that  our  knowledge 
of  its  cause  and  nature  can  grow  but  slowly  at  the  best. 


THE   CORONA.  239 

The  character  of  the  phenomenon  is  such  also  as 
to  make  its  accurate  observation  exceedingly  difficult ; 
slight  differences  in  the  transparency  of  the  atmosphere, 
in  the  sensitiveness  of  the  observer's  eye,  a  preoccupa- 
tion of  the  mind  by  some  feature  which  first  happens 
to  strike  the  attention,  or  a  peculiarity  in  the  manner 
of  representing  what  one  sees,  will  often  make  the  de- 
scriptions and  drawings  of  two  observers,  side  by  side, 
so  discrepant  that  one  would  hardly  imagine  they  could 
refer  to  the  same  object.  For  instance,  in  1870,  two 
naval  officers  on  the  deck  of  the  same  vessel  made 
drawings  of  the  corona,  one  of  which  represented  it  as 
a  six-rayed  star,  while  the  other  showed  it  as  composed 
of  two  ovals  crossing  at  right  angles.  In  1878  the 
writer,  on  comparing  notes  immediately  after  the 
eclipse  with  other  members  of  his  party,  found  that 
about  half  of  them  saw  the  corona  principally  extended 
to  the  east  and  west,  while  the  other  half,  himself 
among  them,  were  just  as  positive  that  it  brushed  out 
mainly  to  the  north  and  south.  The  photographs,  and 
other  data  since  collected,  show  that  the  principal  exten- 
sion was  undoubtedly  along  the  east-and-west  line,  but 
that  there  were  much  better  outlined  streamers,  though 
shorter  and  less  brilliant,  directed  from  the  solar  poles. 
Some  eyes  were  more  impressed  by  definiteness  of  form, 
others  by  size  and  luminosity. 

Obviously,  conclusions  must  be  drawn  from  ocular 
impressions  only  with  the  greatest  caution.  Photo- 
graphs are,  of  course,  more  to  be  trusted,  as  far  as  they 
go ;  but,  even  with  them,  a  slight  difference  in  the 
sensitiveness  of  the  plate,  in  the  exposure,  or  in  the 
development,  will  make  a  great  difference  in  the  result- 
ing picture.  Neither  can  any  photograph  ever  bring 
out  everything  which  is  visible  to  the  eye.  An  ex- 


240 


THE  SUN. 


posure,  sufficient  to  exhibit  well  the  fainter  details,  will 
spoil  the  brighter  features,  and  vice  versa.  Moreover, 
it  may,  and  not  seldom  does,  bring  out  features  that  the 
eye  can  not  see  because  their  light  is  mainly  ultra-violet. 
We  can  do  no  better  than  to  refer  one,  who  is  curious 
to  see  how  various  are  the  representations  of  this  won- 
derful  object,  to  Mr.  Ranyard's  magnificent  work  upon 
the  observations  made  during  total  solar  eclipses,  pub- 
lished as  Volume  XLI  of  the  "  Memoirs  of  the  Royal 
Astronomical  Society  of  Great  Britain."  In  it  he  has 
reproduced  nearly  a  hundred  different  drawings  and 
photographs  of  the  corona,  as  seen  during  the  eclipses 
since  1850.  The  steel  engravings  of  the  eclipses  of 


FIG.  80. 


COBONA  AS  OBSJBBTED  BT  LlAIS  IK  1801. 


THE  CORONA.  241 

1870  and  1871,  based  upon  the  photographs  then  made, 
are  among  the  most  accurate  and  beautiful  representa- 
tions of  the  corona  anywhere  to  be  found.  We  have 
copied  a  few  of  his  woodcuts,  which  give  an  idea  of 
the  more  remarkable  features  of  the  phenomenon,  and 

FIG.  81. 


COBONA  OF  I860.— SECOHI. 

exhibit  the  differences  between  its  character  and  ap- 
pearance on  different  occasions ;  we  have  added  also  a 
picture  of  the  corona  as  seen  in  1878,  in  which  we  have 
combined  the  sketches  of  several  observers  with  our 
own  impressions.  Woodcuts,  however,  are  not  com- 
17 


242 


THE  SUN. 


petent  to  bring  out  the  peculiar  filmy,  nebulous  charac- 
ter of  many  of  the  details,  which  can  be  fairly  repre- 
sented only  by  steel  engraving. 


FIG.  82. 


COBONA  OF  I860.— TEMPEL. 

The  drawing  of  Liais,  Fig.  80,  shows  the  "  petal  "- 
like  forms  which  have  been  noticed  in  the  corona  at 
other  times,  but  seem  to  have  been  especially  prominent 
in  the  eclipse  of  1857.  The  figures  of  the  corona  of 
1860,  by  Secchi  and  Tempel  (Figs.  81,  82),  show  how 
widely  observers  only  a  few  miles  apart  will  differ  in 
their  impressions. 


THE  CORONA. 


243 


The  drawing  of  Grosch  in  1867  (Fig.  83)  is  interest- 
ing in  comparison  with  that  of  1878,  as  showing  the 
state  of  the  corona  at  two  similar  times  of  sun-spot  mini- 
mum. The  long  extensions  of  faint  illumination  in  the 
direction  of  the  sun's  equator  and  the  short  but  vivid 
brushes  in  the  polar  regions  are  notable  in  both. 


COEOMA  or  1867.— GBOSOH. 


Bullock's  picture  of  the  eclipse  of  1868  (Fig.  84) 
shows  a  larger  and  more  irregular  corona  than  usual. 
The  drawing  of  Schott  (Fig.  85),  on  the  other  hand, 
shows  the  corona  of  1869  much  smaller  and  more  brill- 


THlT  SUN. 


iant  than  ordinary,  and  the  writer  can  vouch  for  it  as 
giving  pretty  accurately  the  impression  which  he  him- 
self received  at  the  time. 

Many  of  our  readers,  no  doubt,  have  seen  a  much 
more  impressive  picture  of  the  same  corona,  made  by 
Mr.  Oilman  at  Sioux  City,  and  published  in  the  eclipse 
report  of  the  United  States  Naval  Observatory  (repro- 

Fro.  84. 


COBOHA  OF  1868.— BULLOCK. 


duced  in  Mr.  ProcWs  "Sun").  It  shows  an  exten- 
sive system  of  rifts  and  rays,  which,  if  real  objects, 
escaped  the  notice  of  most  observers — their  visibility, 


THE  CORONA. 


245 


perhaps,  depending  on  the  state  of  the  atmosphere, 
which  is  described  as  slightly  hazy,  but  very  steady,  at 
Mr.  Oilman's  station. 

The  drawings  of  Captain  Tupman  and  Mr.  Foe- 
nander  (Figs.  86,  87)  are  interesting  for  comparison 


FIG.  85. 


COKONA  OF  1869. — SCHOTT. 


with  each  other  and  with  the  photographs  of  the  same 
eclipse  (Fig.  88) ;  and  that  of  the  eclipse  of  1878  (Fig. 
89)  is  remarkable  on  account  of  the  enormous  exten- 
sion of  the  faint  brushes  of  nebulosity  which  were 


246 


THE   SUN. 


traced  to  a  distance  of  6°  or  7°  from  the  sun  by  Pro- 
fessors Langley,  Abbe,  and  Newcomb. 

To  these  pictures  of  the  corona  that  appeared  in  our 
first  edition,  we  add  three  others  that  seem  especially 
worthy  of  reproduction, 

Fig.  90  is  from  a  steel  engraving  which  combines 
the  photographs  of  the  Egyptian  eclipse  of  May  17, 


Fitf.  86. 


COBONA  OF  1871.— CAPTAIN  TUPMAN. 


1882.    Like  Figs.  88  and  92,  it  is  a  typical  "  spot-maxi- 
mum corona."     Attention  is  called  to  the  little  comet 


THE   CORONA. 


247 


in  the  corner  of  the  engraving :  it  was  seen  only  dur- 
ing the  eclipse,  but  seems  to  have  been  a  precursor  of 
the  great  comet  that  appeared  the  following  autumn. 


FIG.  87. 


COBONA  OF  1871.— F0ENANDBB. 


Fig.  91  is  a  direct  reproduction  from  a  beautiful 
negative  of  the  eclipse  of  January  1,  1889,  made  by 
Mr.  Burckhalter,  of  Oakland,  Cal.,  with  an  exposure  of 
one  second.  It  is  a  typical  "  spot-minimum  "  corona, 
the  polar  streamers  being  extremely  fine,  and  the  equa- 
torial extension  enormous ;  this  latter,  however,  is  better 
brought  out  on  negatives  of  longer  exposure. 


248 


THE  SUN. 


Fig.  92  is  from  what,  on  the  whole,  is  the  finest 
photograph  ever  yet  obtained  of  the  corona.  It  was 
made  on  April  16,  1893,  at  Mina  Bronces,  in  Chili,  at 


FIG.  88. 


CORONA  OF  1871. — FROM  PHOTOGRAPHS  OF  MR.  DATIS. 

an  altitude  of  6,600  feet,  by  Professor  Schaeberle,  with 
a  photoheliograph  telescope  of  40  feet  focal  length  and 
5  inches  aperture ;  the  sun's  disk  in  the  original  is  about 
4  inches  in  diameter. 

One  of  the  first  questions  which  suggests  itself  with 
reference  to  the  corona  relates  to  its  location :  is  it  a 
phenomenon  of  the  sun,  of  the  moon,  or  of  our  own 


THE  CORONA. 


249 


atmosphere ;  or  is  it  perhaps  a  mere  optical  effect,  like  a 
rainbow  or  a  halo  ?  If  its  seat  is  in  the  earth's  atmos- 
phere, it  is  of  course  an  affair  of  little  magnitude  or 
importance;  if,  on  the  other  hand,  it  is  really  at  the 
sun,  it  must  be  an  object  of  enormous  dimensions  and 
of  cosmical  significance. 

Kepler,  and  many  astronomers  after  him,  attributed 
it  to  the  atmosphere  of  the  moon,  and  this  continued, 
perhaps,  to  be  the  most  generally  accepted  explanation 

FIG.  89. 


CORONA  or  1878 — FROM  COMBINATION  or  VARIOUS  DRAWINGS. 

until  the  early  part  of  the  present  century,  when  it  wag 
shown  by  many  incontestable  considerations  that  the 
moon  possessed  no  atmosphere  to  speak  of ;  certainly 


350  THE 

none  which  could  account  for  the  observed  facts.  From 
this  time  until  1869  the  weight  of  opinion  seems  to 
have  been  rather  in  favor  of  a  terrestrial  or  purely 
optical  origin  for  the  corona,  though  some  (Professor 
Grant,  among  others,  in  1852,  in  his  "  History  of  Physi- 
cal Astronomy  ")  considered  it  more  probable  that  the 
solar  atmosphere  is  the  real  cause. 

The  question  was  first  settled  in  1869  by  the  obser- 
vations of  Professor  Harkness  and  the  writer,  who, 
•independently,  found  the  spectrum  of  the  corona  to  be 
characterized  by  a  bright  line  in  the  green.  It  was 
identified  by  the  writer,  whose  spectroscope  was  very 
powerful  for  the  date,  as  the  "  1,474 "  line  on  Kirch- 
hoff's  map  of  the  solar  spectrum,  then  generally  used 
for  reference.  The  existence  of  this  bright  line  demon- 
strates the  presence,  in  the  corona,  of  incandescent  gas, 
and  this  of  course  can  only  be  near  the  sun.  Some 
doubt  was  cast  upon  the  observations  at  first,  but  they 
have  since  been  fully  confirmed  ;  and  in  1871  a  differ- 
ent and  more  simple  proof  was  added.  Photographs, 
taken  at  stations  which  were  separated  by  several  hun- 
dred miles,  in  India  and  Ceylon,  showed  precisely  the 
same  details  of  coronal  form  and  structure,  and  are,  by 
themselves  considered,  sufficient  to  demonstrate  that  the 
main  features  of  the  phenomenon  are  independent  of 
our  terrestrial  atmosphere  and  the  accidents  of  the  lunar 
surface.  Of  course,  it  is  not  meant  to  affirm  that  our 
own  atmosphere  has  no  part  in  the  phenomenon,  but  its 
rdle  is  only  secondary.  As  has  been  pointed  out  by 
Mr.  Proctor,  the  observer  at  the  middle  of  an  eclipse  is 
in  the  center  of  an  enormous  shadow,  generally  from 
fifty  to  a  hundred  miles  in  diameter.  If  we  grant  that 
the  air  retains  some  sensible  density  and  power  of  light- 
reflection,  even  at  an  altitude  of  a  hundred  miles,  and 


THE   CORONA. 


251 


assume  for  the  shadow  a  radius  of  only  twenty  miles, 
no  particle  of  air  illuminated  by  sunlight  could,  under 
these  circumstances,  be  found  within  11°  of  the  sun's 
apparent  place  in  the  sky.  If  there  were  no  corona 
truly  solar  in  its  origin,  there  would  therefore  be  around 
the  moon  a  circle  of  intense  darkness,  23°  at  least  in 
diameter :  at  the  edge  of  this  circle  a  faint  illumination 
would  begin,  forming  a  luminous  ring,  something  like 
a  halo,  outside  of  which  the  sky  would  be  lighted  by 
rays  from  an  only  partially  hidden  sun.  Of  course,  this 

FIG.  90. 


COKONA  OP  1882.    EGYPT. 

dark  "  hole  in  the  sky  "  would  be  concentric  with  the 
sun  and  moon  only  at  the  moment  when  the  eclipse 
was  central.  In  the  actual  state  of  things,  the  portion 
of  the  sky  in  the  neighborhood  of  the  sun  is,  of 
course,  illuminated  by  whatever  appendages  of  the  sun 


252  THE 

remain  unhidden  by  the  moon,  and  it  is  this  faint  illumi- 
nation, derived  from  the  corona  and  prominences,  which 
gives  to  the  lunar  disk  its  apparent  solid  rotundity. 

We  have  spoken  of  this  illumination  as  faint,  but 
generally  it  is  considered  to  be  much  stronger  than  that 
of  the  full  moon,  though  there  is  some  difference  of 
opinion  on  the  matter.  There  is  no  doubt  that  in  many 
cases  there  is  abundant  light  for  reading  a  watch-face, 
even  at  the  middle  of  the  totality ;  the  writer,  in  1869, 
found  no  use  for  a  lantern  in  making  notes  or  in  read- 
ing a  micrometer-head.  But  undoubtedly  a  large  por- 
tion of  this  light  is  derived,  not  from  the  corona,  but 
from  the  illuminated  air ;  for,  though  the  observer  him- 
self is  in  darkness,  he  has  in  sight  all  around  the  horizon 
a  sunlit  atmosphere.* 

Undoubtedly  there  is  a  great  difference  between 
different  eclipses  in  respect  to  the  obscurity.  The 
brilliance  of  the  lower  part  of  the  corona — a  narrow 
ring  close  to  the  limb  of  the  sun — is  dazzling ;  but  the 
light  falls  off  very  rapidly.  In  an  eclipse  of  long  du- 
ration, therefore,  when  the  moon's  apparent  diameter 
considerably  exceeds  the  sun's,  the  brighter  portion  of 
the  corona  will  be  covered,  and  the  light  will  be  much 
less  than  in  an  eclipse  occurring  when  the  difference 
between  the  diameters  of  the  sun  and  moon  is  only 
small. 

At  the  eclipse  of  1869  an  attempt  was  made  to 

*  This  is  specially  obvious  if  the  sky  is  covered  with  clouds  of  medi- 
um density.  In  August,  1887,  the  writer  had  the  misfortune  to  occupy 
a  station  (about  120  miles  northeast  of  Moscow),  where  it  was  wholly 
overcast,  and  a  misty  rain  was  falling  much  of  the  time  during  the 
eclipse.  At  the  middle  of  the  eclipse  the  darkness  was  hardly  greater 
than  in  a  heavy  thunderstorm :  the  moment  when  "  totality "  began 
could  not  be  determined  with  any  accuracy  at  all,  and  its  close  was 
doubtful  by  some  seconds.  Fine  print  could  be  read  all  the  time. 


THE   CORONA.  253 

measure  the  darkness  of  the  totality,  as  compared  with 
that  of  night.  The  obscurity  proved  to  be  so  much 
deeper  than  had  been  expected,  that  the  ingenious  in- 
strument which  Professor  Eastman  had  devised  for  the 
purpose  turned  out  inadequate  to  deal  with  it  exactly. 
The  apparatus  consisted  of  a  tube  about  ten  inches  long 
and  two  and  a  half  in  diameter.  At  the  bottom  of  this 
was  painted  a  small  white  star  of  five  points,  with  a 
black  dot  in  the  center,  and  a  black  ring  around  it. 
The  other  end  of  the  tube  was  closed  with  a  so-called 

FIG.  91. 


COKONA  OF  JANTTAKT,  1889.— BUECKHALTEB. 

"  cat's-eye,"  a  square  opening,  the  size  of  which  can  be 
varied  at  will,  by  moving  two  slides  with  a  micrometer- 
screw,  or  rack  and  pinion. 

A  small  tube,  attached  obliquely  to  the  large  one, 
like  a  teapot-nose,  allowed  the  observer  to  look  at  the 
star,  and  the  amount  of  light  from  the  sky  was  then 
measured  by  opening  or  shutting  the  slides  until  the 
dot  and  ring  in  the  center  of  the  star  just  ceased  to  be 


254  THE 

visible.  Not  only  did  the  ring  and  dot  become  invisible 
with  the  whole  aperture  of  the  cat's-eye,  but  the  star 
itself  was  invisible  during  the  totality.  Professor  East- 
man, on  the  whole,  concluded  that  the  general  darkness 
was  on  this  occasion  about  the  same  as  an  hour  or  so 
after  sunset,  when  third-magnitude  stars  first  become 
visible.  The  instrument  was  pointed  at  the  zenith, 
however,  and  not  at  the  corona,  so  that  it  gave  no  direct 
determination  of  the  coronal  light.  Neither  do  the  ob- 
servations of  Mr.  Ross,  in  18YO  (by  which  the  general 
illumination  was  compared  with  the  light  from  a  candle), 
answer  the  purpose  any  better.  And  substantially  the 
same  is  true  of  the  observations  made  at  subsequent 
eclipses ;  especially  in  1886  and  1889. 

One  or  two  attempts  have  been  made  to  compare 
the  shadow  cast  by  the  corona  with  that  produced  by  a 
candle ;  but  the  coronal  shadow  has  always  been  so 
masked  by  the  general  aerial  illumination  as  to  defeat 
the  observation.  One  astronomer  only,  so  far  as  known 
to  the  writer,  has  made  an  estimate  of  the  coronal  light 
based  on  anything  like  a  scientific  foundation.  Belli, 
in  1842,  found  that  the  corona  seemed  to  him  to  give 
as  much  light  as  a  candle  at  a  distance  of  1*8  metre. 
He  was  short-sighted,  so  that  an  object  like  a  candle 
appeared  to  him  as  a  confused  patch  of  light,  and  it 
was  by  taking  advantage  of  this  defect  in  his  vision 
that  he  was  able  to  effect  the  comparison,  which  must, 
however,  have  been  only  very  rough.  Two  weeks  later 
he  compared,  in  the  same  way,  the  full  moon,  at  the 
same  altitude,  with  a  similar  candle,  and  thus  found 
that  the  light  of  the  corona  was  less  than  one  sixth  that 
of  the  moon.  This  comparison,  however,  is  so  unsatis- 
factory in  its  details  that  no  great  weight  can  be  allowed 
it,  and  it  must,  perhaps,  be  still  considered  an  open 


THE  CORONA. 


255 


question  whether  the  light  of  the  corona  is  brighter  or 
not  than  that  of  the  moon. 

The  lower  portions  of  the  coronal  ring,  close  to  the 
sun,  are  usually  much  too  bright  to  be  looked  at  com- 
fortably with  a  telescope  unprovided  with  a  shade-glass  ; 
we  have  on  this  point  the  testimony  of  Biela,  Struve, 
Ranyard,  and  others.  Moreover,  at  a  transit  of  Yenus  or 


CORONA  or  1898. 


Mercury  under  favorable  circumstances,  the  black  disk 
of  the  planet  becomes  visible  before  it  reaches  the  sun. 
Janssen  thus  saw  Yenus  in  18Y4,  and  Langley,  Mercury 
in  18T8.  Of  course,  this  implies  behind  the  planet  a 
background  of  sensible  brightness  in  comparison  with 
the  illumination  of  our  atmosphere.  It  is  generally 
considered  that  a  difference  of  one  sixty-fourth  in  the 


256  THE  SUN. 

brightness  of  two  adjacent  portions  of  a  surface  is  the 
smallest  quantity  perceptible  by  the  eye,  and,  if  so,  the 
corona  must  be  more  than  one  sixty-fourth  as  bright 
as  the  aerial  illumination  at  the  edge  of  the  sun's  disk. 
At  an  eclipse,  also,  the  corona  is  sometimes  seen  several 
seconds,  or  even  minutes,  before  the  beginning  and 
after  the  end  of  totality.  Petit,  in  1860,  reports  seeing 
it  twelve  minutes  (sic)  before  the  disappearance  of  the 
sun,  and  Lockyer,  in  1871,  continued  to  see  it  for  three 
minutes  after  the  sun's  reappearance.  But,  as  has  been 
said  before,  the  light  falls  off  very  rapidly,  and  the  out- 
er portions  of  the  corona  are  of  the  faintest  nebulosity. 
It  is  greatly  to  be  desired  that,  at  the  next  eclipse,  some 
careful  photometric  measurements  should  be  made. 

Apart  from  the  difference  in  the  amount  of  light 
at  different  eclipses,  due  to  the  variation  in  the  moon's 
diameter,  there  is  a  strong  probability  that  the  corona 
itself  changes  considerably  in  brightness  and  extent 
from  year  to  year.  In  1878  it  was  the  general  verdict 
of  the  numerous  observers,  who  had  also  seen  the  eclipse 
of  1869,  that  the  corona  was  much  less  brilliant  than 
on  the  former  occasion.  Still,  several  observers  of 
deservedly  high  reputation  hold  a  precisely  contrary 
opinion.  The  corona  of  1878  was  unquestionably  the 
more  extensive. 

Of  course,  the  known  facts  as  to  the  periodicity  of 
sun-spots,  and  the  sympathy  between  them  and  the 
prominences,  make  it  antecedently  probable  that  a  cor- 
responding variation  will  be  found  in  the  corona. 

In  the  eclipses  of  1867r,  187r8,  and  1889,  all  of  which 
occurred  near  the  sun-spot  minimum,  the  corona  was 
characterized  by  very  long,  faint  equatorial  extensions, 
with  distinctly  defined  diverging  polar  rays:  on  the 
other  hand,  in  1870,  1882,  and  1893,  the  equatorial 


THE   CORONA.  257 

wings  and  polar  rays  were  much  less  striking,  the  corona 
was  more  nearly  circular,  and  its  principal  development 
was  over  the  sun-spot  zones.  Figs.  89  and  92  may  be 
taken  as  nearly  typical. 

In  the  eclipse  of  1878,  which  occurred  at  a  sun-spot 
minimum,  the  spectroscopic  peculiarities  of  the  corona 
were  also  greatly  modified.  The  bright  line,  which  is 
its  principal  characteristic,  became  so  faint  that  many 
observers  missed  it  altogether. 

This  bright  line,  as  has  been  said  before,  was  first 
recognized  as  coronal  at  the  eclipse  of  1869.  It  had 
been  seen  reversed  in  the  spectrum  of  the  chromosphere 
a  few  weeks  previously,  both  by  Mr.  Lockyer,  and,  in- 
dependently, by  the  writer,  who,  however,  did  not  know 
of  the  earlier  observation  until  some  time  after  the 
eclipse.  In  the  ordinary  solar  spectrum  it  appears  as  a 
fine,  dark  line  at  1,474  of  Kirchhoff's  scale,  or  5,316-9 
of  Rowland's — a  line  in  no  way  conspicuous  as  com- 
pared with  hundreds  of  others,  and  barely  visible  with 
a  single-prism  spectroscope.  "With  a  spectroscope  of 
high  dispersion  it  was  found,  in  1876,  to  be  closely 
double,  the  upper  (more  refrangible)  component  being 
slightly  hazy,  while  the  other  is  sharp  and  well-defined. 
The  upper  component  is  the  true  coronal  line,  and  is 
always  seen  without  much  difficulty,  reversed  in  the 
spectrum  of  the  chromosphere.  Both  Kirchhoff  and 
Angstrom  give  the  line  as  belonging  to  the  spectrum 
of  iron,  a  fact  which  was  for  a  time  very  perplexing, 
since  it  is  hardly  possible  that  the  vapor  of  this  metal 
could  really  be  the  prevailing  constituent  of  the  corona, 
surmounting  even  hydrogen  itself.  This  difficulty,  how- 
ever, no  longer  exists,  for  it  is  now  clear  that  the  iron- 
line  is  the  lower  component  of  the  double,  its  close 
proximity  to  the  other  being  only  accidental.  The 
18 


258 


SUN. 


figure  gives  a  representation  of  the  line  and  its  sur- 
roundings, as  seen  in  a  high-dispersion  spectroscope. 
The  scale  above  the  spectrum  is  that  of  Angstrom. 


FIG.  98. 


PORTION  OF  THK  SPECTRUM  NEAR  THE  CORONA  LINE  (05), 
as  seen  with  an  instrument  of  high  dispersion. 

The  hydrogen-lines  and  H  and  K  also  appear  bright 
in  the  corona-spectrum.  It  is,  perhaps,  not  quite  certain 
that  this  may  not  be  due  to  reflection  of  the  light  of  the 
chromosphere  in  our  own  atmosphere,  but,  on  the  whole, 
probably  not.*  The  atmospheric  reflection  extends  in- 
ward, at  an  eclipse,  over  the  dark  disk  of  the  moon,  as 
well  as  outward,  and  if  the  appearance  of  the  hydrogen- 
lines  were  due  simply  to  this  reflection,  they  should  be 
just  as  strong  on  the  moon's  disk  as  in  the  corona.  This 
does  not  seem  to  be  the  case,  though  in  1870  the  writer 
saw  them  plainly  on  the  center  of  the  lunar  disk ;  but 
Janssen  and  Lockyer  agree  that  they  are  much  brighter 
outside.  The  "  1,474  line  "  has  been  traced,  by  an  ana- 
lyzing spectroscope,  on  some  occasions  to  an  elevation 
of  nearly  20'  above  the  moon's  limb,  and  the  hydrogen- 
lines  nearly  as  far.  What  is  important  also,  the  lines 
were  just  as  strong  in  the  middle  of  a  dark  rift  as  any- 
*  See  Note  C,  page  359. 


THE   CORONA.  259 

where  else.  "We  shall  have  occasion  to  recur  to  this 
again. 

With  the  analyzing  spectroscope  the  1,474  line  is 
very  much  feebler  near  the  sun's  limb  than  the  hydro- 
gen-lines, i.  e.,  taking  any  small  portion  of  the  corona 
near  the  limb,  the  hydrogen  is  much  more  brilliant  than 
the  unknown  vapor  which  produces  the  other  line. 
When,  however,  the  eclipse  is  examined  by  an  integrat- 
ing spectroscope,*  the  relation  of  brightness  is  reversed, 
showing  that  the  total  amount  of  "  1,474  light"  is  the 
greater,  and  indicating  either  that  it  comes  from  a  much 
more  extensive  area,  or  else  that  in  the  upper  regions 
the  hydrogen  loses  its  brightness  much  more  rapidly 
than  the  other  material. 

As  to  the  substance  which  produces  the  1,474  line 
we  have  no  knowledge  as  yet,  though  the  name  "  coro- 
nium"  has  been  provisionally  assigned  to  it,  and  the 
recent  probable  identification  of  "  helium  "  in  terrestrial 
minerals  gives  strong  reason  to  hope  that  before  very 
long  we  may  find  coronium  also.  It  would  seem  that  it 
must  be  something  with  a  vapor  density  below  that  of 
hydrogen  itself,  which  is  incomparably  the  lightest  of 
all  bodies  now  known  to  our  terrestrial  chemistry.  It 
can  hardly  be  any  one  of  our  familiar  elements,  even  in 
any  allotropic  modification,  such  as  has  been  suggested 
by  some,  for,  in  the  midst  of  the  most  violent  disturb- 
ances which  are  observed  sometimes  in  prominences  and 
near  sun-spots,  when  the  lines  of  hydrogen,  magnesium, 
and  other  metals  are  contorted  and  shattered  by  the 
swiftness  of  the  rush  of  the  contending  elements,  this 
line  usually  remains  undisturbed,  fine,  sharp,  and 
straight ;  a  little  brightened,  but  not  otherwise  af- 
fected. For  the  present  it  stands  (as  did  the  helium 

*  See  pages  72,  73  for  explanation  of  this  term. 


260  THE  SUN. 

lines  until  Ramsay'.s  discovery)  an  unexplained  mys- 
tery.* 

Besides  this  line  and  the  hydrogen-lines,  two  others 
have  been  doubtfully  reported  in  the  greenish-yellow 
part  of  the  spectrum.  One  of  them  seems  to  have  been 
seen  twice :  first,  in  1869  by  the  writer,  and  in  1870  by 
Denza,  in  Italy.  Its  place  is  about  5,570  of  Angstrom's 
scale.  Still,  as  one  of  the  barium-lines,  which  is  fre- 
quently and  brilliantly  reversed  in  the  spectrum  of  the 
chromosphere,  is  not  very  far  from  this  place  (at  5,534), 
it  is  quite  possible  that  this  was  the  line  seen.  The 
other  doubtful  line  (reported  by  the  writer  in  1869)  was 
at  5,450  (Angstrom),  also  very  near,  in  fact  between, 
the  places  of  two  lines  which  are  conspicuous  in  the 
chromosphere. 

The  photographic  spectrum  of  the  corona,  observed 
more  or  less  fully  at  every  eclipse  since  1882,  is  full  of 
detail  and  interest.  Its  most  striking  feature  is  the 
great  calcium  pair,  H  and  K,  but  the  violet  and  ultra- 
violet lines  of  hydrogen  are  also  conspicuous,  and  there 
are  a  multitude  of  others  less  so.  It  is  not  easy,  how- 

*  Its  frequent  identification  with  a  line  in  the  spectrum  of  the  aurora 
borealis,  for  which,  unfortunately,  the  writer  was  at  first  mainly  respon- 
sible, is  a  striking  example  of  the  difficulty  of  correcting  a  mistake  which 
has  once  gained  currency.  A  few  weeks  before  the  first  discovery  of  this 
line  in  the  spectrum  of  the  corona,  Professor  Winlock  had  observed 
the  spectrum  of  a  bright  aurora,  and  had  published  the  position  of  five 
lines :  one  of  the  five  positions  coincides  with  that  of  the  1,474  line  far 
within  the  limits  of  error  probable  in  such  an  observation,  and  I  jumped 
to  the  conclusion  that  the  coincidence  was  exact  and  significant.  Later 
observations  soon  showed  that  this  "  line  "  in  the  aurora  spectrum  is  not 
a  line  at  all,  strictly  speaking,  but  a  faint,  hazy  band,  never  to  be  seen 
except  in  unusually  bright  auroras,  and  not  at  all  identifiable  with  the 
1,474  line  of  the  corona.  So  far  as  the  spectroscope  goes,  there  is  no 
indication  of  any  connection  between  the  corona  and  the  aurora  of  the 
earth's  atmosphere,  though  there  are  other  facts  which  suggest  that  the 
phenomena  may  be  to  some  extent  similar  in  their  nature. 


THE  CORONA.  261 

ever,  to  discriminate  between  those  lines  that  are 
truly  coronal  and  those  that  belong  to  the  chromo- 
sphere.* It  would  be  well  at  the  first  opportunity  to 
obtain  photographs  with  simple  "integrating"  spectro- 
scopes as  well  as  with  "  analyzing  "  instruments. 

Besides  bright  lines,  the  corona  shows  also  a  faint 
continuous  spectrum,  and  in  this  Janssen  and  Barker 
have  observed  a  few  of  the  more  prominent  dark  lines 
of  the  solar  spectrum — D,  5,  and  G  especially. 

This  fact  of  course  shows  that  while  the  corona  may 
be  in  great  part  composed  of  glowing  .gas,  as  indicated 
by  the  bright  lines  of  its  spectrum,  it  also  contains  a 
considerable  quantity  of  matter  in  such  a  state  as  to  re- 
flect the  sunlight — matter,  probably,  in  the  form  of  dust 
or  fog. 

This  conclusion  is  borne  out  also  by  the  result  of 
observations  with  different  forms  of  polariscope,  which, 
for  the  most  part,  indicate  that  the  light  of  the  corona 
is  partially  polarized  in  radial  planes,  just  as  it  should 
be  if  in  part  composed  of  reflected  light.  We  have  said 
"for  the  most  part,"  because  there  have  been  some 
very  puzzling  discrepancies  between  different  instru- 
ments and  different  observers,  which  we  have  not  space 
to  discuss  here. 

Since  the  corona,  then,  contains  both  incandescent 
gas  and  also  matter  in  such  a  condition  of  mist  or  smoke 
as  fits  it  to  reflect  light,  it  is  an  interesting  question 
whether  different  parts  of  the  coronal  structure  are  com- 
posed alike  of  both,  or  whether  there  is  a  separation. 

It  has  been  attempted  to  solve  the  question  by  ex- 
amining the  eclipse  with  a  so-called  "  slitless  spectro- 
scope " — i.  e.,  simply  a  prism  put'  in  front  of  the  object- 
glass  of  a  small  telescope.  If,  with  such  an  instrument, 
one  were  to  look  at  a  distant  object  emitting  homoge- 

*  See  Note  C,  page  357. 


262  THE  SUN. 

neous  light  (an  alcohol-flame  tinged  with  salt,  for  in- 
stance), one  would  see  it.  precisely  as  if  the  prism  were 
not  there,  except  that  the  refraction  would  change  the 
apparent  direction  of  the  object.  If  the  light  were 
composed  of  three  or  four  bright  lines,  like  that  from  a 
Geissler  tube  filled  with  hydrogen,  there  would  then 
appear  the  same  number  of  colored  images.  If  the 
light  were  like  that  of  an  ordinary  candle,  which  gives 
a  continuous  spectrum,  one  would  get  merely  a  colored 
streak.  Finally,  if  we  had  a  source  of  light  combining 
these  different  conditions,  a  lamp-flame,  for  instance, 
tinged  in  some  parts  with  sodium  and  in  others  with 
lithium,  we  should  then  have  the  streak  of  color  marked 
in  the  yellow  with  a  clear  image  of  the  sodium  part  of 
the  flame,  and  in  the  red  and  violet  with  images  of  that 
part  of  the  flame  which  was  colored  by  lithium. 

If,  then,  the  long  rays  and  streamers  of  the  corona 
were  mainly  composed  of  the  gas  which  gives  the  1,474 
line,  we  ought  to  see  them  distinctly  through  the  prism 
on  a  background  produced  by  the  light  from  the  reflect- 
ing mist.  Nothing  of  the  kind  occurs,  however.  The 
slitless  spectroscope,  in  the  hands  of  various  observers 
since  1870,  has  shown  a  continuous  band  of  light  with 
several  smooth,  bright  rings  upon  it :  the  brightest  and 
largest  ring  was  green  (corresponding  to  the  1,474  line), 
and  there  were  three  other  fainter  ones  in  the  red,  blue> 
and  violet,  corresponding  to  the  three  brightest  lines  of 
hydrogen.  It  is  to  be  inferred,  therefore,  that  the  gas- 
eous matter  of  the  corona  forms  a  pretty  regular  atmos- 
phere around  the  sun,  and  that  the  structural  elements, 
the  rays,  rifts,  and  streamers,  are  mainly  due  to  mist  or 
dust — at  least  they  seem  to  give  a  continuous  spectrum. 
With  this  agrees  the  fact,  before  mentioned,  that  the 
1,474  line  is  just  as  bright  in  the  middle  of  one  of  the 


THE   CORONA.  263 

dark  rifts  as  in  a  bright  streamer.  In  18T8  the  slitless 
spectroscope,  however,  failed,  in  the  hands  of  all  the 
observers,  to  show  any  rings  at  all.  This  fact,  taken 
with  the  lessened  brightness  of  the  corona  on  that  occa- 
sion, seems  to  indicate  that  the  gases  of  the  coronal  at- 
mosphere, at  the  time  of  a  sun-spot  minimum,  are  much 
diminished  in  extent  and  brilliance,  while  the  streamers 
are  comparatively  unaffected. 

RAPID    CHANGES    IN    THE    CORONA. 

The  question  has  been  often  raised,  whether  the 
appearance  of  the  corona  changes  during  an  eclipse. 
Many  drawings  seem  to  show  that  this  is  the  case ;  they 
represent  the  corona  at  the  beginning  and  end  of  the 
eclipse  as  much  wider  on  that  side  of  the  sun  less  deeply 
covered  by  the  moon — on  the  western  edge,  near  the 
beginning  of  the  eclipse,  and  on  the  eastern,  near  its  end 
— while  it  is  approximately  symmetrical  at  the  middle 
of  totality  ;  and  this  circumstance  was  much  relied  upon 
for  a  time  by  those  who  maintained  that  the  corona  is, 
in  the  main,  a  phenomenon  of  the  earth's  atmosphere. 
Other  drawings,  however,  of  the  same  eclipses,  show 
nothing  of  the  kind,  nor  do  the  photographs,  except  in 
one  or  two  instances,  where  a  sufficient  explanation  is  to 
be  found  in  drifting  clouds.  On  the  other  hand,  pho- 
tographs taken  at  different  moments  during  an  eclipse, 
and  at  stations  many  hundred  miles  apart,  agree  so  close- 
ly as  to  make  it  evident  that  the  main  features  of  the 
corona  change  only  gradually,  persisting,  as  a  rule,  for 
hours  at  least,  and  perhaps  for  days  and  weeks  for  aught 
we  know.  At  the  same  time  they  do  sometimes  change 
perceptibly,  even  in  the  course  of  twenty  minutes,  while 
the  shadow  is  traveling  between  stations  only  a  few 
hundred  miles  apart.  Some  have  thought  they  saw 


264  THE  SUN. 

rapid  movements  in  the  streamers,  and  have  described 
them  as  waving  and  flickering ;  one  or  two  have  even 
imagined  that  the  corona  "  whirled  like  a  Catherine- 
wheel."  Probably  this  is  mere  imagination,  though 
the  unsteadiness  of  the  air  might  give  a  person  unused 
to  astronomical  observation  the  idea  of  quivering  mo- 
tion. The  usual  impression  upon  the  mind  is  quite 
different — that  of  calm,  serene  stability. 

Combining  the  facts  that  have  been  ascertained,  and 
speaking  in  the  most  general  way,  it  would  seem  that 
the  corona  is  mainly  composed  of  filaments  which  either 
emanate  from  the  sun  or  are  developed  in  his  atmos- 
phere most  abundantly  at  those  portions  of  his  surface 
about  midway  between  the  equator  and  the  poles,  those 
filaments  which  are  emitted  on  either  side  of  the  zone 
having  a  tendency  to  lean  toward  the  central  ones.  As 
a  consequence,  the  corona  tends  toward  the  form  of  a 
four-rayed  star,  the  points  of  which  are  inclined  45°  to 
the  sun's  axis,  and  are  made  up  of  converging  filaments, 
constituting  the  synclinal  structure  which  Mr.  Ranyard 
first  clearly  brought  out. 

Obviously,  however,  this  statement  must  be  taken 
very  loosely.  Every  eclipse  presents  striking  excep- 
tions. There  are  always  streamers  tangential,  curved, 
or  inclined,  which  can  be  brought  under  no  such  rule ; 
faint,  far-reaching  cones  of  light,  like  those  which  were 
seen  in  1878 ;  dark  rifts,  rounded  masses  of  nebulosity, 
vortices,  and  a  multitude  of  other  peculiarities  of  struct- 
ure no  more  reducible  to  a  formula  than  the  shapes  of 
flame  or  cloud. 

Opinion  is  very  widely  divided  as  to  the  nature 
and  origin  of  the  substances  which  compose  the  coronal 
structures.  Very  few  now,  we  think,  deny  the  pres- 
ence of  an  atmosphere  of  incandescent  gases  reaching 


THE  CORONA.  265 

to  an  elevation  of  at  least  300,000  miles,  and  this  al- 
though there  are  enormous  difficulties  in  harmonizing 
an  atmosphere  of  such  extent  with  the  low  pressure  at 
the  surface  of  the  photosphere,  indicated  by  the  fineness 
of  the  Fraunhofer  lines  in  the  spectrum.  But,  as  to 
the  material  of  which  the  streamers  are  composed,  and 
the  nature  of  the  forces  which  determine  their  form 
and  position,  their  is  no  agreement.  Some  see  in  the 
corona  simply  flocks  of  meteors,  and  there  can  be  no 
doubt  that  meteoric  matter  must  abound  in  the  sun's 
immediate  neighborhood.  But  looking,  for  instance,  at 
the  pictures  of  the  eclipse  of  1871  and  1889  it  appears 
evident  that  the  details  of  that  corona  could  not  be  ac- 
counted for  in  this  way.  It  seems  much  more  likely 
that  the  phenomena  of  comets'  tails  and  the  streamers  of 
the  aurora  are  phenomena  of  the  same  order,  and  though 
as  yet  the  establishment  of  this  relation  would  not 
amount  to  anything  like  an  explanation  of  the  corona, 
it  would  be  a  step  toward  it — a  step  by  no  means  taken 
yet,  however,  it  must  be  admitted  ;  nor  is  it  easy  to  see 
at  present  how  the  problem  is  to  be  attacked.  That  the 
forces  concerned  reside  in  the  sun  himself  is  made  prob- 
able by  the  usual  approximate  symmetry  of  the  corona 
with  reference  to  his  axis,  and  the  fact  that  the  coronal 
streamers  seem  to  originate  most  abundantly  nearly  in 
the  sun-spot  zones. 

But  we  must  evidently  wait  awhile  for  the  solution 
of  the  problems  presented  by  the  beautiful  phenomenon. 
Possibly  the  time  may  come  when  some  new  contrivance 
may  enable  us  to  see  and  study  the  corona  in  ordinary 
daylight,  as  we  now  do  the  prominences.  The  spectro- 
scope, indeed,  will  not  accomplish  the  purpose,  since 
the  rays  and  streamers  of  the  corona  give  a  continuous 
spectrum ;  but  it  would  be  rash  to  say  that  no  means 


266  THE  SUN. 

will  ever  be  found  for  bringing  out  the  structures  around 
the  sun  which  are  hidden  by  the  glare  of  our  atmos- 
phere. Unless  something  like  this  can  be  done,  the 
progress  of  our  knowledge  must  probably  be  very  slow, 
for  the  corona  is  visible  only  about  eight  days  in  a  cent- 
ury, in  the  aggregate,  and  then  only  over  narrow  stripes 
on  the  earth's  surface,  and  but  from  one  to  five  minutes 
at  a  time  by  any  one  observer.* 

Within  the  past  few  years  a  number  of  very  earnest 
attempts  have  been  made  in  the  direction  indicated. 
Dr.  Huggins  was  the  first  to  move,  and  from  1883  for 
a  number  of  years  worked  hard  in  the  endeavor  to  ob- 
tain photographs  of  the  corona  in  full  sunshine.  He 
succeeded  very  early  in  getting  a  number  of  plates  show- 
ing around  the  sun  certain  faint,  elusive  halo-forms 
which  certainly  look  very  coronal.  Plans  were  made, 
and  were  carried  out,  in  1884,  for  using  a  similar  appa- 
ratus upon  the  Riffelberg  in  Switzerland,  and  after- 
ward at  the  Cape  of  Good  Hope.  Nothing  has  been 
obtained,  however,  much  in  advance  of  Dr.  Huggins's 
own  first  results.  But  since  September,  1883,  until  late 
in  1885,  the  air,  as  every  one  knows,  was  full  of  a  fine 
haze,  probably  composed  in  the  main  of  dust  and  vapor 
from  Krakatoa,  which  greatly  interfered  with  all  such 
operations. 

About  the  same  time  that  Dr.  Huggins  was  photo- 
graphing in  England,  Professor  Wright,  of  New  Haven, 
was  experimenting  on  the  same  subject  in  a  different 
way.  He  reflected  the  sun's  rays  into  a  darkened  room 

*  This  estimate  is  based  upon  the  fact  that  total  eclipses  occur  on  the 
average  about  once  in  two  years,  that  the  shadow  occupies  (on  the  aver- 
age, again)  some  three  hours  in  traversing  the  globe,  and  that  the  mean 
duration  of  totality  is  between  two  and  three  minutes,  never  by  any  pos- 
sibility reaching  eight  minutes,  and  very  seldom  six. 


THE   CORONA. 

by  a  heliostat,  cut  out  all  but  the  blue  and  violet  rays  by 
a  suitable  absorbing-cell,  and  then  formed  an  image  of 
the  sun  and  its  surroundings  upon  a  sensitive  fluorescent 
screen,  stopping  out  the  sun's  disk  itself.  He  obtained 
on  the  screen,  on  more  than  one  occasion,  what  he  then 
believed  and  still  believes  to  be  a  true  image  of  the 

O 

corona.  But  the  aerial  haze  soon  intervened  to  put  an 
end  to  all  such  operations ;  for  of  course  it  is  evident 
that  success,  whether  by  photography  or  by  fluorescence, 
is  possible  only  under  conditions  of  unexceptionable  at- 
mospheric purity. 

Both  Professor  Wright  and  Dr.  Huggins  base  their 
hopes  upon  the  belief,  which  seems  to  be  warranted  by 
the  spectrum-photographs  obtained  during  the  Egyptian 
eclipse  of  May,  1882,  that  the  light  of  the  corona  and  of 
the  upper  regions  of  the  sun's  "atmosphere"  (if  one 
may  so  speak  of  what  is  not  strictly  an  "  atmosphere  " 
at  all)  is  peculiarly  rich  in  violet  and  ultra-violet  rays 
— that  the  corona  is  far  more  brilliant  to  the  photo- 
graphic plate  and  to  the  fluorescent  screen  than  to  the 
eye. 

The  reports  from  the  eclipse  of  August  29, 1886,  ob- 
served by  English  and  American  parties  on  the  island 
of  Grenada  in  the  Southern  "West  Indies  were  strongly 
unfavorable  to  the  reality  of  the  coronal  appearances 
obtained  by  Huggins  and  Wright  in  their  attempts  to 
render  the  corona  visible  without  an  eclipse.  Plates 
furnished  by  Mr.  Huggins,  and  precisely  similar  to  those 
which  he  has  employed  in  his  photographic  experiments, 
were  exposed  by  Captain  Darwin  during  the  totality  (as 
well  as  before  and  after  it),  in  an  apparatus  like  Mr. 
Huggins's,  with  a  time  of  exposure  the  same  that  he  has 
been  using,  and  were  treated  and  developed  in  accord- 
ance with  his  directions.  The  plates  which  were  thus 


268  THE 

exposed  during  the  totality  show  no  corona  at  all,  the 
exposure  time  having  proved  insufficient  to  bring  it  out. 
ISTor  do  the  plates  exposed  during  the  partial  phase  show 
any  trace  of  the  moon's  outline  beyond  the  sun's  limb. 
Of  course  this  makes  it  extremely  probable  that  what 
looks  like  the  corona  upon  plates  exposed  in  the  same 
way  to  the  uneclipsed  sun  is  merely  a  fallacious  ghost, 
due,  as  his  opponents  have  always  claimed,  to  something 
in  his  apparatus  or  process,  or  else  to  the  scattering  of 
light  in  our  atmosphere.  It  is  true,  as  Mr.  Common 
points  out,  that  the  result  is  not  absolutely  conclusive, 
because  the  air  was  by  no  means  satisfactorily  clear  dur- 
ing the  eclipse ;  but  it  must  be  conceded,  and  Mr.  Hug- 
gins  himself  admits  it,  that  the  probability  is  now  heavi- 
ly against  him.  Captain  Darwin  obtained  good  pictures 
of  the  corona  with  ordinary  plates  exposed  for  a  longer 
time  in  the  usual  apparatus. 

The  later  eclipses  of  1889  and  1893  also  bear  in  the 
same  direction. 

More  recently  still,  Professor  Hale  has  made  a  new 
attempt  with  the  spectro -heliograph,  from  Pike's  Peak 
and  the  top  of  ^Etna,  as  well  as  from  his  own  observa- 
tory. He  was  in  hopes  that  by  the  use  of  the  double 
slit  of  the  instrument,  shutting  out  all  but  the  "  K  light," 
which  is  especially  strong  in  the  spectrum  of  the  corona, 
the  effect  of  the  aerial  illumination  might  be  relatively 
reduced  to  a  great  degree,  because  in  the  spectrum  of 
the  air  light,  "  K  light "  is  almost  wanting — in  the  air 
spectrum  K  is  a  black  band.  But  he  has  had  no  better 
success  than  his  predecessors. 

He  is  now  about  to  try  still  another  method,  and 
endeavor  to  make  the  corona  manifest  itself  by  its  heat- 
rays,  using  for  the  purpose  a  bolometric  apparatus  very 
similar  to  that  with  which  Professor  Langley  has  been 


THE  CORONA.  269 

making  his  remarkable  investigation  of  the  infra-red 
spectrum. 

Probably  it  must  be  admitted  that  at  present  the 
predominant  opinion  among  astronomers  and  photog- 
raphers is  against  the  practicability  of  reaching  the 
corona  without  an  eclipse,  by  any  such  methods ;  still, 
to  the  writer  at  least,  the  case  appears  by  no  means  ab- 
solutely hopeless,  and  success  is  certainly  devoutly  to  be 
desired. 

We  must  not  close  the  chapter  without  a  few  words 
as  to  the  recent  course  and  the  present  state  of  theoret- 
ical speculation  respecting  the  corona. 

At  the  eclipse  of  1883,  observed  on  Caroline  Island, 
in  the  Pacific  Ocean,  by  French  and  American  parties, 
Professor  Hastings  made  observations  for  the  purpose 
of  testing  a  theory  he  had  framed,  that  the  outlying  re- 
gions of  the  corona  are  merely  a  diffraction  effect  pro- 
duced by  the  edge  of  the  moon ;  the  diffraction  being 
not  that  due  to  the  regular  periodicity  of  light-vibra- 
tions, ordinarily  discussed,  but  due  to  the  probable  con- 
tinually occurring  discontinuity  or  change  of  phase  in 
the  vibrations.  It  seems  probable,  from  a  not  perfectly 
complete  investigation,  that  such  discontinuity  might 
scatter  light  far  beyond  the  limits  of  ordinary  diffrac- 
tion. He  found  during  the  eclipse,  by  an  apparatus 
constructed  expressly  for  the  purpose,  that  the  bright 
corona-line  (1474  K)  was  always  visible  to  a  much 
greater  distance  from  the  sun  on  the  side  least  deeply 
covered  by  the  moon  than  on  the  other,  as  unquestion- 
ably ought  to  be  the  case  if  his  theory  were  correct. 

But  the  same  thing  would  result  from  the  diffusion 
of  light  by  the  air ;  and  the  French  observers,  and  nearly 
all  others  who  have  discussed  the  matter,  feel  satisfied 
that  this  is  the  true  explanation  of  what  he  saw.  He 


270  THE 

himself  now,  we  understand,  thinks  it  not  impossible 
that  a  thin  cloud  may  have  passed  over  the  sun  just  at 
the  critical  moment,  and  so  have  vitiated  his  observa- 
tion. 

The  discussion  which  has  followed  his  publication 
seems  to  have  onl j  strengthened  the  older  view,  that  the 
corona  is  a  true  solar  appendage,  an  intensely  luminous 
though  inconceivably  attenuated  cloud  of  gas,  fog,  and 
dust,  surrounding  the  sun,  formed  and  shaped  by  solar 
forces. 

The  fact  that  comets,  themselves  mere  airy  nothings, 
have  several  times  (the  last  instance  was  in  1882)  passed 
absolutely  through  the  corona  without  experiencing  any 
sensible  disturbance  of  path  or  structure,  Jias,  however, 
been  always  felt  by  many  as  an  almost  insuperable  diffi- 
culty with  this  accepted  theory,  and  more  than  anything 
else  led  Professor  Hastings  to  propose  his  new  hypothe- 
sis. But,  on  careful  consideration,  we  shall  find  that 
our  conceptions  of  the  possible  attenuation  of  shining 
matter  near  the  sun  will  bear  all  the  needed  "  stretch- 
ing" without  involving  any  absurdity.  Recalling  the 
phenomena  of  the  electrical  discharge  in  Crookes's  tubes, 
it  is  clear  that  a  "  cloud,"  with  perhaps  only  a  single 
molecule  to  the  cubic  foot  (but  thousands  of  miles  in 
thickness),  would  answer  every  luminous  condition  of 
the  phenomena.  And  all  the  rifts  and  streamers,  and 
all  the  peculiar  structure  and  curved  details  of  form,  cry 
out  against  the  diffraction  hypothesis. 

Professor  Schaeberle,  of  the  Lick  Observatory,  has 
proposed  a  very  different  hypothesis,  which  he  calls  a 
"  mechanical "  theory  of  the  solar  corona.  As  a  basis 
he  assumes  that  the  eruptions  from  the  sun's  surface  are 
most  active  and  numerous  in  the  spot-zones,  and  that 
the  sun  rotates  upon  an  axis  inclined  82f  °  to  the  plane 


THE   CORONA.  271 

of  the  earth's  orbit.  Then,  in  his  own  words,  "  the  theo- 
retical corona  is  caused  by  light  emitted  and  reflected 
from  streams  of  matter  ejected  from  the  sun  by  forces 
which  in  general  act  along  lines  normal  to  the  surface 
of  the  sun,  these  forces  being  most  active  near  the  cen- 
ter of  each  sun-spot  zone." 

Many  of  the  apparent  variations  in  the  type  of  the 
corona  will  therefore  depend  upon  the  perspective  un- 
der which  these  streams  are  seen,  according  to  the  time 
of  year ;  others  upon  the  relative  abundance  and  force 
of  the  streams  on  different  portions  of  the  solar  surface 
according  to  the  phase  of  the  sun-spot  period  at  the 
time;  and  others  yet,  the  curved  rays  especially,  are 
caused  by  optical  illusions  due  to  the  apparent  crossing 
and  interlacing  of  streams  that  lie  in  different  planes. 

If  we  assume,  as  Mr.  Schaeberle  does,  that  the  ejected 
material  which  forms  the  streamers  leaves  the  sun  with 
a  velocity  which  may  be  as  great  as  nearly  four  hundred 
miles  a  second,  and  returns  with  the  same  velocity  after 
having  traveled  as  far  away  as  the  orbits  of  Jupiter  and 
Saturn,  it  appears  that  the  neighborhood  of  the  sun 
ought  to  be  filled  with  a  diffuse  shower  of  swiftly  de- 
scending dust,  met  and  penetrated  by  the  more  definite 
and  concentrated  ascending  jets.  In  the  interplay  and 
conflict  of  the  rising  and  falling  materials  Mr.  Schae- 
berle thinks  he  finds  the  explanation  of  the  periodicity 
of  the  sun-spots,  while  he  accounts  for  the  existence  of 
the  sun-spot  zones  by  the  manner  in  which  the  heated 
gases,  rising  from  the  center  of  the  cooling  globe  of  the 
sun,  would  reach  the  surface  and  produce  in  the  photo- 
sphere zones  of  greater  and  lesser  thickness — belts  of 
surface  strength  and  weakness.  For  a  more  detailed 
account  of  this  theory,  which  has  already  gained  pretty 
wide  acceptance,  we  must  refer  the  reader  to  its  pro- 


272  THE  SUN. 

poser's  original  papers  in  the  Lick  Observatory  report 
upon  the  eclipse  of  December,  1889,  and  in  Vol.  xiii 
of  "  Astronomy  and  Astro-Physics,"  April,  1894.  It 
gives,  however,  no  intelligible  account  of  the  apparent 
spectroscopic  differences  between  the  chromosphere  and 
corona,  and  the  magnetic  forces  acting  at  the  sun  do  not 
appear  in  it  at  all. 

What  may  be  considered  as  the  principal  rival  theory 
regards  the  corona  as  very  like  a  permanent  "  aurora  " 
around  the  sun,  the  position  and  direction  of  its  stream- 
ers being  determined  by  the  sun's  magnetic  field  of 
force,  in  the  same  way  that  the  terrestrial  lines  of 
magnetic  force  direct  the  beams  of  our  own  "aurora 
borealis." 

Professor  Bigelow,  of  the  U.  S.  Signal  Service,  has 
recently  investigated  the  subject  mathematically,  and 
apparently  with  great  success  so  far  as  concerns  the  dis- 
tribution, curvature,  and  general  appearance  of  the 
streams  and  filaments  which  compose  the  corona.  He 
finds  that  in  the  sun,  as  in  the  earth,  the  magnetic  axis 
does  not  coincide  with  the  axis  of  rotation,  the  sun's 
north  magnetic  pole  being  distant  4J°,  and  the  southern 
9£°,  from  the  corresponding  pole  of  rotation.  He  finds 
that  in  the  corona  of  the  eclipse  of  1878  (the  photo- 
graphs of  which  he  was  able  to  obtain  for  measurement) 
the  force  which  directs  the  streamers  appears  to  be  re- 
pulsive, and  that  the  bases  of  the  individual  streamers, 
not  very  numerous  but  of  enormous  dimensions,  are 
mainly  grouped  in  a  zone  some  10°  wide,  with  the 
greatest  density  about  34°  from  the  coronal  poles,  while 
their  visible  upper  extremities  are  located  about  500,- 
000  miles  vertically  above  the  sun-spot  belts.  He  adds : 
"  At  this  place  the  incandescence  of  the  material  parti- 
cles apparently  ceases,  and  if  condensation  sets  in  there 


THE  CORONA.  273 

would  exist  the  conditions  required  for  the  precipitation 
of  cool  masses,  whose  fall  upon  the  surface  of  the  sun  is 
generally  supposed  to  produce  the  spots."  He  considers 
that  the  "  structureless  equatorial  wing  is  no  doubt  a 
floating  mass  of  matter  cooling  in  the  process  of  prepa- 
ration for  precipitation."  For  further  details  we  must 
refer  the  reader  to  Professor  Bigelow's  papers  in  the 
"American  Journal  of  Science"  from  1891  to  1894. 

As  to  the  origin  of  the  repulsive  force  which  causes 
the  projection  of  the  streamers  from  the  polar  regions, 
Professor  Bigelow  does  not  commit  himself,  though 
evidently  having  in  view  the  idea  that  it  may  be  "  elec- 
trical "  in  the  broad  sense  of  the  word. 

We  must  not  close  the  chapter  without  at  least  a 
reference  to  the  beautiful  experiments  of  Dr.  Pupin,  of 
New  York.  Under  certain  conditions  he  obtains  mag- 
nificent "coronoidal  discharges"  from  the  surface  of 
a  brass  ball  inclosed  within  a  large  glass  globe,  from 
which  the  air  has  been  more  or  less  perfectly  exhausted. 
The  photographs  are  certainly  very  suggestive,  and  seem 
to  show  that  if  there  are  violent  electric  disturbances 
upon  the  sun — "  solar  thunderstorms,"  so  to  speak — 
they  might  inductively  produce  coronal  streamers.  The 
possibility,  however,  of  such  thunderstorms  at  solar  tem- 
peratures is  extremely  doubtful  at  present,  and  the  sub- 
ject must  be  left  for  further  development. 


CHAPTER  VIII. 

TffE  SUITS  LIGHT  AND  HEAT. 

Sunlight  expressed  in  Candle-Power. — Method  of  Measurement. — Bright* 
.  ness  of  the  Sun's  Surface.  —  Langley's  Experiment.  —  Diminution 
of  Brightness  at  Edge  of  the  Sun's  Disk. — Hastings's  View  as  to 
Nature  of  the  Absorbing  Envelope. — Total  Amount  of  Absorption  by 
Sun's  Atmosphere. — Thermal,  Luminous,  and  Actinic  Rays:  their 
Fundamental  Identity  and  Differences. — Measurement  of  the  Sun's 
Radiation. — Herschel's  Method. — Expressions  for  the  Amount  of 
Sun's  Heat. — Pouillet's  Pyrheliometer. — Crova's. — Violle's  Actinom- 
eter. — Langley's  Researches. — Absorption  of  Heat  by  Earth's  Atmos- 
phere ;  by  the  Sun's. — Question  as  to  Differences  of  Temperature  on 
Different  Portions  of  Sun's  Disk. — Question  as  to  Variation  of  Sun's 
Radiation  with  Sun-Spot  Period. — The  Sun's  Temperature — Actual 
— Effective. — Views  of  Secchi,  Ericsson,  Pouillet,  Vicaire,  Rosetti, 
Le  Chatelier,  and  Wilson  and  Gray. — Schemer's  Spectroscopic  Evi- 
dence.— Evidence  from  the  Burning-Glass. — Langley's  Experiment 
with  the  Bessemer  "  Converter." — Permanency  of  Solar  Heat  for  last 
Two  Thousand  Years. — Meteoric  Theory  of  Sun's  Heat. — Helmholtz's 
Contraction  Theory. — Possible  Past  and  Future  Duration  of  the  Sun's 
Supply  of  Heat. — Siemens's  Untenable  Theory. 

SUNLIGHT  is  the  intensest  radiance  at  present  known. 
It  far  exceeds  the  brightness  of  the  calcium-light,  and 
is  not  rivaled  even  by  the  most  powerful  electric  arc. 
Either  of  these  lights  interposed  between  the  eye  and 
the  surface  of  the  sun  appears  as  a  black  spot  upon  the 
disk. 

We  can  measure  with  some  accuracy  the  total  quan* 


THE  SUN'S  LIGHT  AND  HEAT.  275 

tity  of  sunlight,  and  state  the  amount  in  "  candle-pow- 
er " ;  the  figure  which  expresses  the  result  is,  however,  so 
enormous  that  it  fails  to  convey  much  of  an  idea  to  the 
mind— it  is  1,575,000,000,000,000,000,000,000,000 :— fif- 
teen hundred  and  seventy-five  billions  of  billions,  enu- 
merated in  the  English  manner,  which  requires  a  mil- 
lion million  to  make  a  billion  ;  or  one  octillion  five  hun- 
dred and  seventy-five  septillion,  if  we  prefer  the  French 
enumeration. 

The  "  candle-power,"  which  is  the  unit  of  light  gen- 
erally employed  in  photometry,*  is  the  amount  of  light 
given  by  a  sperm-candle  weighing  one  sixth  of  a  pound, 
and  burning  a  hundred  and  twenty  grains  an  hour. 
An  ordinary  gas-burner,  consuming  five  feet  of  gas 
hourly,  gives,  if  the  gas  is  of  standard  quality,  from 
twelve  to  sixteen  times  as  much  light.  The  total  light 
of  the  sun  is  therefore  about  equivalent  to  one  hundred 
billion  billion  of  such  gas-jets. 

This  statement  rests  mainly  upon  the  measurements 
made  by  Bouguer  in  1725,  and  Wollaston  in  1799 ;  since 
then,  however,  confirmed  by  others.  They  found  that 
the  sun  in  the  zenith  would  illuminate  a  white  surface 
about  sixty  thousand  times  as  intensely  as  a  standard 
candle  at  the  distance  of  one  metre.  Allowing  for  ab- 
sorption of  light  in  our  atmosphere,  the  figure  would 
rise  to  about  seventy  thousand.  As  the  distance  of 
the  sun  is  ninety-three  million  miles,  or  very  nearly  a 
hundred  and  fifty  million  kilometres,  it  follows  that, 
if  we  multiply  70,000  by  the  square  of  150,000,000,000 
(reducing  kilometres  to  metres),  the  product  will  express 

*  The  photometric  unit  proposed  by  the  Paris  International  Congress 
in  1890  is  one  twentieth  of  the  light  emitted  by  a  square  centimetre  of 
molten  platinum  just  solidifying.  It  is  called  the  "  decimal  candle,"  and 
is  about  one  per  cent,  smaller  than  the  old  unit. 


2Y6 


THE   SUN. 


the  number  of  candles  which,  at  the  sun's  distance, 
would  give  a  light  equal  to  that  of  the  sun.  The  num- 
ber comes  out  as  stated  above,  though  it  is  undoubt- 
edly uncertain  by  a  considerable  percentage.  It  de- 
pends upon  old  observations,  which  ought  to  be  re- 
peated ;  observations,  also,  which  are  difficult  and 
never  very  satisfactory  because  of  the  vagueness  of 
the  unit,  the  extreme  difference  between  the  intensity 
of  the  lights  compared,  and,  what  is  still  more  trouble- 
some, the  difference  between  the  color  of  the  sun- 
light and  of  candle-light. 

Fio.  94. 


METHOD  or  MEASURING  THE  INTENSITY  OP  SUNLIGHT. 

The  method  of  making  such  a  comparison  is  illus- 
trated by  Fig.  94.  A  mirror,  J/,  throws  the  rays  of  the 
sun  into  a  darkened  room  upon  a  small  lens,  the  diam- 
eter of  which  is  accurately  known.  This  lens  brings 


THE  SUN'S  LIGHT  AND   HEAT. 

the  rays  to  a  focus  at  F,  after  passing  which  point  they 
diverge  and  fall  upon  a  white  screen,  /S}  at  a  consider- 
able distance.  Neglecting  for  the  present  the  loss  of 
light  by  reflection  at  the  surface  of  the  mirror  and 
transmission  through  the  lens,  we  may  say  that  the 
illumination  of  the  screen  is  as  many  times  less  than 
that  of  full  sunlight  as  the  area  of  the  lens  L  is  less  than 
that  of  the  whole  disk  of  light  upon  the  screen.  If,  for 
instance,  the  lens  is  one  fourth  of  an  inch  in  diameter, 
and  the  circle  of  light  on  the  screen  is  ten  feet  across, 
then  the  light  on  the  screen  would  be  230,400  times 
fainter  than  sunlight.  If  we  allow  for  the  loss  by  re- 
flection and  in  the  lens,  the  ratio  would  probably  not 
be  far  from  300,000  to  1.  Of  course,  these  two  correc- 
tions must  be  (and  can  be)  accurately  determined  by 
special  observations  for  the  purpose.  Having  got  thus 
far,  there  are  various  methods  of  proceeding.  The 
simplest,  and  by  no  means  the  least  accurate,  is  to  place 
a  small  rod,  like  a  pencil,  near  the  screen,  so  that  its 
shadow  will  be  cast  by  the  sunlight  at  a :  the  candle  of 
comparison,  (7,  is  then  moved  back  and  forth  until  a 
position  is  found  at  which  the  shadow  cast  by  its  flame 
at  5  is  equally  strong  with  the  other  shadow.  Then  the 
relative  amounts  of  illumination  on  the  screen  produced 
by  the  sun  and  by  the  candle  will  be  as  the  squares  of 
the  lines  a  F  and  b  C.  There  are  other  methods  ad- 
mitting of  still  greater  precision,  but  all  embarrassed 
(as  this  is)  by  the  difference  of  color  between  sun-  and 
candle-light.  The  most  uncertain  part  of  the  operation 
lies,  however,  in  the  corrections  for  loss  of  light  in  the 
atmosphere,  at  the  mirror,  and  in  the  lens. 

Thus  far  we  have  considered  only  the  total  light 
emitted  by  the  sun.  The  question  of  the  intrinsic 
brightness  of  his  surface  is  a  different  though  connected 


278  THE  SUN. 

one,  depending  for  its  solution  upon  the  same  observa- 
tions, combined  with  a  determination  of  the  light-radi- 
ating areas  in  the  different  cases.  Since  a  candle-flame 
at  the  distance  of  one  metre  looks  considerably  larger 
than  the  disk  of  the  sun,  it  is  evident  that  it  must  be  a 
good  deal  more  than  seventy  thousand  times  less  brill- 
iant. In  fact,  it  would  have  to  be  at  a  distance  of 
about  1*65  metres  to  cover  the  same  area  of  the  sky  as 
the  sun  does,  and  therefore  the  solar  surface  must  ex- 
ceed by  a  hundred  and  ninety  thousand  times  the  aver- 
age brightness  of  the  candle-flame. 

In  the  calcium-light  the  luminous  point  is  both 
much  more  brilliant  and  much  smaller  than  a  candle- 
flame,  so  that  the  discrepancy  is  considerably  less.  Ac- 
cording to  certain  experiments  by  Foucault  and  Fizeau 
in  18M,  the  solar  surface  was  found  to  be  a  hundred 
and  forty-six  times  more  brilliant  than  the  incandescent 
lime.  At  the  same  time  they  experimented  upon  the 
electric  arc,  and  found  the  brightest  part  of  this  to  be 
only  about  four  times  fainter  than  the  sun.  Their  ex- 
periments were,  however,  conducted  by  exposure  of  a 
Daguerreotype-plate  to  the  rays  to  be  compared,  and 
there  is  room  for  considerable  doubt  as  to  their  accuracy. 
Later  experiments  have  showed  in  some  cases  a  rather 
higher  intensity  for  the  brightness  of  the  positive  car- 
bon of  the  electric  arc  (which  is  always  much  more  brill- 
iant than  the  negative).  It  is  asserted  in  a  few  instances 
to  have  reached  a  brilliance  fully  half  as  great  as  that 
of  the  solar  surface ;  but  the  evidence  is  not  entirely 
satisfactory,  the  comparisons  being  only  indirect.  The 
magnificent  lights  produced  by  the  dynamo  -  electric 
machines  of  the  present  day  differ  from  that  employed 
by  Foucault  and  Fizeau,  not  so  much  in  intensity  as  in 
quantity.  The  illuminating  surfaces  are  larger,  and  the 


THE   SUN'S  LIGHT  AND  HEAT.  279 

extent  of  the  arc  much  greater,  but  the  brightness  of 
the  luminous  points  concerned  seems  to  remain  about 
the  same,  and  probably  depends  mainly  upon  the  phys- 
ical characteristics  of  the  carbon,  which  are  essentially 
the  same  in  all  cases. 

One  of  the  most  interesting  observations  upon  the 
brightness  of  the  sun  is  that  of  Professor  Langley,  who, 
a  few  years  ago  (in  1878),  made  a  careful  comparison 
between  the  solar  radiation  and  that  from  the  blinding 
surface  of  the  molten  metal  in  a  Bessemer  "  converter." 
The  brilliance  of  this  metal  is  so  great  that  the  dazzling 
stream  of  melted  iron,  which,  at  one  stage  of  the  pro- 
ceedings, is  poured  in  to  mix  with  the  metal  already  in 
the  crucible,  "  is  deep  brown  by  comparison,  presenting 
a  contrast  like  that  of  dark  coffee  poured  into  a  white 
cup."  The  comparison  was  so  conducted  that,  inten- 
tionally, every  advantage  was  given  to  the  metal  in 
comparison  with  the  sunlight,  no  allowances  being  made 
for  the  losses  encountered  by  the  latter  during  its  pas- 
sage through  the  smoky  air  of  Pittsburg  to  the  reflector 
which  threw  its  rays  into  the  photometric  apparatus. 
And  yet,  in  spite  of  all  this  disadvantage,  the  sunlight 
came  out  five  thousand  three  hundred  times  brighter 
than  the  dazzling  radiance  of  the  incandescent  metal. 

Thus  far  we  have  spoken  of  the  sun  as  a  whole,  but, 
as  has  been  said  before,  there  is  a  marked  diminution 
of  the  light  at  the  edges  of  the  disk ;  so  marked,  indeed, 
that  it  is  exceedingly  surprising  that  any  person  should 
ever  have  questioned  the  fact,  as  some — Lambert,  for 
instance — have  done.  Arago  came  very  near  it,  for  he 
set  the  difference  at  only  -fa — so  little  as  to  be  hardly 
perceptible.  An  image  of  the  sun  a  foot  in  diameter, 
formed  by  a  small  telescope  of  two  inches'  aperture, 
upon  a  white  paper  screen,  shows  the  fact,  however,  in 


280  THE  SUN. 

an  entirely  unquestionable  manner.  Many  measure- 
ments have  been  made  for  the  purpose  of  comparing 
the  brightness  of  different  parts  of  the  disk.  Professors 
Pickering  and  Langley,  in  this  country,  and  Vogel,  in 
Germany,  are  among  the  most  recent  and  reliable  inves- 
tigators of  the  subject.  Professor  Pickering  effected 
his  measurements  by  forming,  with  a  small  telescope, 
an  image  of  the  sun,  about  sixteen  inches  across,  upon  a 
white  screen  perforated  with  an  orifice  three  fourths  of 
an  inch  in  diameter.  The  telescope  was  placed  horizon- 
tally, and  the  light  directed  upon  it  by  a  mirror,  much  as 
in  the  preceding  figure,  except  that  the  mirror  was  moved 
by  clock-work,  so  as  to  keep  the  image  constantly  in  one 
place.  After  the  rays  passed  the  orifice  in  the  screen 
they  were  received  upon  the  disk  of  a  Bunsen  pho- 
tometer, and  the  light  compared  with  that  of  a  standard 
candle,  in  the  ordinary  way,  and  thus  the  ratio  was 
found  between  the  brilliance  of  the  center  of  the  disk 
and  that  of  other  parts.  Pickering  makes  the  ratio 
between  the  intensity  of  the  light  from  the  edge  and 
center  to  be  thirty-seven  per  cent. 

Yogel,  in  1877,  proceeded  still  more  elaborately. 
His  instrument,  called  a  spectral  photometer,  enabled 
him  to  compare  with  great  accuracy,  and  directly,  the 
brightness  of  the  rays  of  different  colors  proceeding  from 
different  parts  of  the  sun — the  red  rays  by  themselves, 
and  the  same  with  the  yellow,  green,  blue,  and  violet. 
The  following  table  contains  an  abridgment  of  his  re- 
sults. In  the  first  column,  headed  D,  is  given  the  distance 
of  the  point  from  the  sun's  center  in  percentage  of  the 
sun's  radius.  The  other  columns  give  the  ratio  between 
the  light  of  the  given  color  at  the  center  of  the  disk 
and  at  the  point  in  question,  expressed  also  as  a  per- 
centage. Thus,  at  the  very  edge  of  the  disk,  at  a  dis- 


THE  SUN'S  LIGHT  AND  HEAT. 


281 


tance  of  one  hundred  per  cent,  of  the  sun's  radius  from 
its  center,  the  violet  light  has  an  intensity  of  only  thir- 
teen per  cent,  of  its  intensity  at  the  center,  and  the  red 
thirty  per  cent,  of  its  central  intensity : 


D. 

Violet, 
A  408. 

Blue, 
A  470. 

Green, 
A  512. 

Yellow, 
A  589. 

Bed, 
662. 

Pickering, 
general  light. 

0 

100 

100 

100 

100 

100 

100 

10 

99-6 

99-7 

99'7 

99-8 

99-9 

98-8 

20 

98-5 

98-8 

98-7 

99-2 

99-5 

.... 

30 

96-3 

97'2 

96-9 

98-2 

98-9 

40 

93-4 

94-1 

94-3 

96-7 

980 

940 

60 

88-7 

91-3 

90-7 

94-5 

96'7 

91-3 

60 

82-4 

87-0 

86-2 

90-9 

94-8 

87-0 

70 

74-4 

80-8 

80-0 

84-5 

91-0 

.... 

75 

69-4 

76-7 

75-9 

80-1 

88-1 

78-V 

80 

63-7 

71-7 

70-9 

74-6 

84-3 

.... 

85 

56-7 

65-5 

64'7 

67-7 

79-0 

69-2 

90 

47'7 

57-6 

56-6 

59-0 

71-0 

.... 

95 

34-7 

45'6 

44-0 

46-0 

58'0 

55'4 

100 

13-0 

16-0 

18-0 

25-0 

30-0 

37-4 

We  have  added,  in  a  last  column,  some  of  the  results 
of  Professor  Pickering,  which,  it  will  be  seen,  for  the 
most  part  are  in  quite  satisfactory  accordance  with  those 
of  Yogel. 

One  thing  is  obvious  from  Vogel's  table,  namely, 
that  the  color  of  the  light  must  be  different  at  the  edge 
of  the  disk  from  what  it  is  in  the  center,  since  more  of 
the  violet  light  than  of  the  red  is  lost  at  the  limb. 

Professor  Langley,  in  1875,  in  attempting  to  meas- 
ure directly  the  relative  brightness  of  points  near  the 
center  and  limb  by  bringing,  in  a  very  ingenious  man- 
ner, the  light  from  the  two  points  to  confront  each 
other  on  a  Bunsen  photometer-disk,  found  this  to  be  a 
very  noticeable  fact — the  edge  is  of  a  sort  of  chocolate- 
brown  and  the  center  quite  bluish,  if  we  take  ordinary 
sunlight  as  the  standard  of  whiteness.  The  difference 
of  tint  was  sufficiently  decided  to  make  the  measures 


THE  STJN. 

very  difficult.  We  have  never  seen  in  print  the  results 
of  this  work  of  his,  and  do  not  know  whether  they  have 
yet  been  published.  Yogel's  work,  however,  from  the 
greater  completeness  of  its*  analysis  in  respect  to  the 
different  colors,  must  take  the  precedence  of  everything 
hitherto  done  in  this  line. 

The  cause  of  this  enf  eeblement  of  the  light  near  the 
limb  of  the  sun  is,  of  course,  the  absorption  of  a  portion 
of  the  rays  by  the  solar  atmosphere.*  It  becomes, 
therefore,  an  interesting  subject  of  inquiry,  how  much 
of  the  sunlight  is  thus  absorbed — how  much  brighter 
the  sun  would  shine  if  suddenly  stripped  of  its  gaseous 
envelopes  ? 

Unfortunately,  the  question  does  not,  in  the  present 
state  of  science,  admit  of  a  certain  and  definite  answer. 
By  making  certain  assumptions  as  to  the  constitution 
of  the  luminous  surface  and  the  character  of  the  atmos- 
phere we  may,  it  is  true,  deduce  mathematical  formulae 

*  It  has  generally  been  considered  that  this  absorbing  envelope  must 
be  gaseous,  and  it  has  usually  been  identified  with  the  so-called  reversing 
layer.  Professor  Hastings,  of  New  Haven,  has,  however,  proposed  a  some- 
what different  theory,  viz.,  that  the  absorption  is  produced  by  matter 
in  a  pulverulent  condition,  at  a  lower  temperature  than  the  photospheric 
clouds,  and  disseminated  through  the  lower  portions  of  the  sun's  true 
atmosphere.  He  urges  with  force  that  the  absorption  of  gases,  at  such  a 
temperature,  must  be  selective,  producing  bands  and  lines  in  the  spectrum, 
while  the  absorption  with  which  we  have  to  do  in  this  case  is  general^ 
simply  weakening  all  the  rays  pretty  much  alike,  though  of  course  affect- 
ing those  of  short-wave  length  more  than  those  of  long,  as  previously 
pointed  out  by  Langley.  The  substance  concerned,  he  says,  must  be  one 
which  condenses  and  precipitates  at  a  temperature  higher  than  that  of 
the  photosphere,  so  that  its  vapor  would  not  be  present  to  any  appreci- 
able extent  in  the  photosphere  and  reversing  layer,  and  its  lines  would 
not  be  found  in  the  solar  spectrum.  He  suggests  that  the  substance  is 
very  probably  carbon,  the  lines  of  which,  at  the  time  he  wrote,  had  not 
been  detected,  though  since  discovered  by  Lockyer  and  Rowland.  It  is 
difficult  at  present  to  determine  its  real  identity. 


THE   SUN'S  LIGHT  AND   HEAT.  283 

(of  a  rather  complicated  character)  which  will  represent 
the  observed  facts  on  those  assumptions. 

Laplace,  for  instance,  assumed  that  each  point  upon 
the  luminous  surface  of  the  sun  radiated  equally  in  all 
directions,  and  that  its  atmosphere  was  homogeneous 
throughout — knowing,  of  course,  that  it  could  not  be 
homogeneous,  but  not  knowing  what  laws  of  density 
and  temperature  would  apply  in  the  case,  and  therefore 
not  being  able  to  supply  a  more  correct  hypothesis. 
On  these  assumptions,  and  taking  as  a  basis  of  calcula- 
tion the  observations  of  Bouguer,  which  in  the  main 
agree  with  the  more  modern  ones,  he  found  that  the 
solar  atmosphere  must  absorb  about  eleven  twelfths  of 
the  whole  light ;  in  other  words,  that  the  sun,  without 
its  atmosphere,  would  be  about  twelve  times  as  bright 
as  we  see  it  now.  Secchi  has  also  adopted  his  conclu- 
sion. 

His  first  assumption,  however,  is  probably  very  far 
from  true.  So  far  as  we  know,  no  luminous  surface 
behaves  as  he  supposes,  but  generally  the  radiations  at 
an  oblique  angle  are  vastly  less  powerful  than  those 
perpendicular  to  the  surface.  According  to  Laplace's 
assumption,  the  sun,  without  its  atmosphere,  would  be 
much  brighter  at  the  edge  than  at  the  center.  Now,  an 
incandescent  sphere  of  metal,  or  an  illuminated  globe  of 
white  glass  (like  the  shade  of  a  student-lamp),  appears 
sensibly  of  equal  brightness  all  over,  the  foreshortening 
of  each  square  inch  of  surface  inclined  to  the  line  of 
eight  just  compensating  for  its  diminished  radiation. 
Assuming  this  law  of  radiation  for  the  solar  surface, 
and  still  keeping  the  hypothesis  of  a  homogeneous  at- 
mosphere, Professor  Pickering  shows  that  the  observed 
darkening  from  the  center  to  the  edge  of  the  sun's  disk, 
indicated  by  his  measures,  would  be  accounted  for  pretty 


284  THE  SUN. 

accurately  by  supposing  this  atmosphere  to  have  a  height 
approximately  equal  to  the  sun's  radius,  and  of  such 
absorbent  power  as  to  reduce  the  light  by  about  seventy- 
four  per  cent,  at  the  center  of  the  disk,  leaving  twenty- 
six  per  cent,  to  pass.  From  this  it  is  possible  to  show 
that  the  whole  light,  if  there  were  no  solar  atmosphere, 
would  be  about  four  and  two  thirds  times  as  great  as 
now — always,  be  it  remembered,  accepting  the  assump- 
tions. 

Yogel,  assuming  the  same  fundamental  law  of  radia- 
tion, finds  from  his  observations  that  the  removal  of  the 
solar  atmosphere  would  increase  the  brightness  of  its 
red  rays  about  1'49  times,  and  of  the  violet  3-01.  The 
difference  between  this  result  and  that  of  Pickering  is 
larger  than  would  be  expected  from  the  general  near 
accordance  of  the  observations,  but  is  probably  princi- 
pally due  to  the  fact  that  Yogel  employs  a  formula  of 
Laplace's  which  implicitly  assumes  the  solar  atmosphere 
to  be  very  thin  as  compared  with  the  size  of  the  sun 
itself,  while  Pickering's  method  of  calculation  accepts 
no  such  limitation.  There  is  an  important  difference 
also  between  the  observations  of  the  two  investigators 
near  the  edge  of  the  disk  :  Yogel's  observations  show  a 
much  more  rapid  degradation  of  the  light  just  there, 
and  so  indicate  an  atmosphere  much  denser,  but  of  less 
elevation  than  Pickering's. 

It  is  evident,  however,  that  for  the  present  we  must 
content  ourselves  with  the  rather  vague  statement  that 
the  removal  of  the  sun's  atmosphere  would  multiply  its 
brightness  several  times.  It  is  almost  certain  that  the 
amount  of  light  received  by  the  earth  would  be  doubled  ; 
it  is  hardly  likely  that  it  would  be  quintupled.  More- 
over, its  color  would  be  materially  changed,  and  its  tint, 
as  pointed  out  by  Langley,  would  be  more  Hue  than 


THE  SUN'S  LIGHT   AND   HEAT.  285 

now.  The  solar  atmosphere  reddens  the  light  trans- 
mitted through  it,  in  just  the  same  way  that  our  terres- 
trial atmosphere  does  at  sunset,  but  to  a  less  degree. 

Thus  far  we  have  confined  ourselves  to  those  radia^ 
tions  which  affect  the  sense  of  vision.  But  these  rays 
do  more :  if  received  upon  a  dark  surface  they  are,  as 
we  say,  u  absorbed,"  and  the  absorbing  body  becomes 
warmer.  Nothing  in  science  is  now  much  more  certain 
than  that  these  luminous  radiations  consist  of  pulses  of 
inconceivable  (but  measurable)  frequency,  which  are 
communicated  through  intervening  space ;  pulses  which 
are  capable  not  merely  of  affecting  the  visual  nerves  of 
sentient  beings,  but  of  producing  also  many  other  effects, 
physical,  thermal,  or  chemical,  according  to  the  surface 
which  receives  them.  The  human  eye,  however,  is  very 
circumscribed  in  its  range  of  perception,  taking  cogni- 
zance only  of  such  vibrations  as  do  not  exceed  or  fall 
short  of  certain  limits  of  frequency — the  slowest  oscilla- 
tions it  recognizes  being  those  of  the  extreme  red,  which 
number  about  three  hundred  and  ninety  millions  oi 
millions  of  vibrations  in  a  second ;  while  the  most  rapid, 
those  of  the  extreme  violet,  are  nearly  twice  as  frequent, 
making  seven  hundred  and  seventy  millions  of  millions 
in  the  same  time.  The  rays  emitted  by  the  sun  are  not, 
however,  so  limited ;  but  the  visual  vibrations  are  accom- 
panied by  others  both  many  times  more  slow  and  more 
rapid.  There  has  been  a  prevailing  idea  for  many  years, 
founded  upon  Brewster's  fallacious  experiments,  that 
thermal,  luminous,  and  chemical  rays  are  fundamentally 
different,  though  coexistent  in  the  sun's  beams.  Thia 
is  erroneous.  It  is  true,  indeed,  that  rays  whose  vibra- 
tions are  too  slow  to  be  seen  produce  powerful  heating 
effects,  and  that  those  which  are  invisible  because  they 
are  too  rapid  have  a  strong  influence  in  determining 


286  THE 

certain  chemical  and  physical  reactions ;  but  it  is  also 
true  that  the  visible  rays  are  capable  of  producing  the 
same  effects  to  a  greater  or  less  degree,  and  there  is 
some  reason  for  thinking  that  certain  animals  can  see 
by  rays  to  which  the  human  retina  is  insensible.  There 
is  absolutely  no  philosophical  basis  for  distinction  be- 
tween the  visible  and  invisible  radiations  of  the  sun, 
except  in  the  one  point  of  vibration-frequency — their 
pitch,  to  use  the  analogy  of  sound.  The  expressions 
thermal,  luminous,  and  chemical  rays  are  apt  to  be  mis- 
leading. All  the  waves  of  solar  radiation  are  carriers  of 
energy,  and  when  intercepted  do  work,  producing  heat, 
or  vision,  or  chemical  action,  according  to  circumstances. 

If  the  amount  of  solar  light  is  enormous,  as  com- 
pared with  terrestrial  standards,  the  same  thing  is  still 
more  true  of  the  solar  heat,  which  admits  of  somewhat 
more  accurate  measurement,  since  we  are  no  longer 
dependent  on  a  unit  so  unsatisfactory  as  the  "  candle- 
power,"  and  can  substitute  thermometers  and  balances 
for  the  human  eye. 

It  is  possible  to  intercept  a  beam  of  sunshine  of 
known  dimensions,  and  make  it  give  up  its  radiant 
energy  to  a  weighed  mass  of  water  or  other  substance, 
to  measure  accurately  the  rise  of  temperature  produced 
in  a  given  time,  and  from  these  data  to  calculate  the 
whole  amount  of  heat  given  off  by  the  sun  in  a  minute 
or  a  day. 

Pouillet  and  Sir  John  Herschel  seem  to  have  been 
the  first  fairly  to  grasp  the  nature  of  the  problem,  and 
to  investigate  the  subject  in  a  rational  manner. 

HerschePs  experiments  were  made  in  1838  at  the 
Cape  of  Good  Hope,  where  he  was  then  engaged  in  his 
astronomical  work.  He  proceeded  in  this  way:  A 
small  tin  vessel,  containing  about  half  a  pint  of  water, 


THE  SUN'S  LIGHT  AND  HEAT.  287 

carefully  weighed,  was  placed  on  a  light  wooden  sup- 
port, touching  it  at  only  three  points.  This  was  put 
inside  of  a  considerably  larger  cylinder,  also  of  tinned 
iron,  this  outer  cylinder  having  a  double  cover  with  a 
hole  in  it,  the  cover  large  enough  to  shade  the  sides  of 
the  vessel,  and  the  hole  a  little  less  than  three  inches  in 
diameter.  A.  delicate  thermometer  was  immersed  in 
the  water,  with  a  sort  of  dasher  of  mica  for  the  purpose 
of  stirring  it  and  keeping  the  temperature  uniform 
throughout  the  mass.  The  apparatus  was  so  placed  and 
adjusted  that  the  whole  of  the  light  and  heat  passing 
through  the  hole  in  the  cover  would  fall  upon  the  sur- 
face of  the  water,  the  sun  at  that  time  (December  31st) 
being  within  12°  of  the  zenith  at  noon. 

This  apparatus  was  placed  in  the  sunshine  and  al- 
lowed to  stand  for  ten  minutes,  shaded  by  an  umbrella, 
and  the  slight  rise  in  the  temperature  of  the  water  was 
noted.  Then  the  umbrella  was  removed  and  the  solar 
rays  were  allowed  to  fall  upon  the  water  for  the  same 
length  of  time,  and  the  much  larger  rise  of  temperature 
was  noted.  Finally,  the  apparatus  was  again  shaded, 
and  the  change  for  ten  minutes  again  observed.  The 
mean  between  the  effects  in  the  first  and  last  ten-minute 
intervals  could  be  taken  as  the  measure  of  the  influence 
of  other  causes  besides  the  sun,  and  deducting  this  from 
the  rise  during  the  ten  minutes'  insolation,  we  have  the 
effect  of  the  simple  sunshine. 

HerschePs  figures  for  his  first  experiment  run  as 
follows : 

Rise  of  temperature  in  first  ten  minutes 0'°25 

"    "  "  tt  second  ten  minutes  (sun) 3'°90 

"     "  «  "  third  ten  minutes O'°10 

The  mean  of  the  first  and  third  is  O°17,  and  this  de- 
ducted from  the  second  gives  3*°73  as  the  rise  of  tern* 


288  THE  SUN. 

perature  produced  by  a  sunbeam  three  inches  in  diam- 
eter, absorbed  by  a  mass  of  matter  equivalent  to  4,638 
grains  of  water  (we  do  not  indicate  the  minutiae  of  the 
process  by  which  the  weight  of  the  tin  vessel,  ther- 
mometer, stirrer,  etc.,  are  allowed  for).  Nothing  more 
is  now  necessary  to  enable  us  to  compute  just  how  much 
heat  is  received  by  the  earth  in  a  day  or  a  year,  except, 
indeed,  the  determination  of  the  very  troublesome  and 
somewhat  uncertain  correction  for  the  absorption  of 
heat  by  the  earth's  atmosphere — a  correction  deduced 
by  means  of  observations  made  at  varying  heights  of 
the  sun  above  the  horizon. 

Herschel  preferred  to  express  his  results  in  terms 
of  melting  ice,  and  put  it  in  this  way :  the  amount  of 
heat  received  on  the  earth's  surface,  with  the  sun  in 
the  zenith,  would  melt  an  inch  thickness  of  ice  in  two 
hours  and  thirteen  minutes  nearly. 

Since  there  is  every  reason  to  believe  that  the  sun's 
radiation  is  equal  in  all  directions,  it  follows  that,  if  the 
sun  were  surrounded  by  a  great  shell  of  ice,  one  inch 
thick  and  a  hundred  and  eighty-six  million  miles  in 
diameter,  its  rays  would  just  melt  the  whole  in  the 
same  time.  If,  now,  we  suppose  this  shell  to  shrink  in 
diameter,  retaining,  however,  the  same  quantity  of  ice 
by  increasing  its  thickness,  it  would  still  be  melted  in 
the  same  time.  Let  the  shrinkage  continue  until  the 
inner  surface  touches  the  photosphere,  and,  allowing 
for  atmospheric  absorption,  the  ice -envelope  would 
become  more  than  a  mile  thick,  through  which  the 
solar  fire  would  still  thaw  out  its  way  in  the  same  time 
— at  the  rate,  according  to  Herschel's  determinations, 
of  more  than  forty  feet  a  minute.  Herschel  continues 
that,  if  this  ice  were  formed  into  a  rod  45-3  miles  in 
diameter,  and  darted  toward  the  sun  with  the  velocity 


THE   SUN'S  LIGHT  AND   HEAT.  289 

of  light,  its  advancing  point  would  be  melted  off  as  fast 
as  it  approached,  if  by  any  means  the  whole  of  the  solar 
rays  could  be  concentrated  on  the  head.  Or,  to  put  it 
differently,  if  we  could  build  up  a  solid  column  of  ice 
from  the  earth  to  the  sun,  nearly  two  miles  and  a  half 
in  diameter,  spanning  the  inconceivable  abyss  of  ninety- 
three  million  miles,  and  if  then  the  sun  should  concen- 
trate his  power  upon  it,  it  would  dissolve  and  melt,  not 
in  an  hour,  nor  a  minute,  but  in  a  single  second :  one 
swing  of  the  pendulum,  and  it  would  be  water ;  seven 
more,  and  it  would  be  dissipated  in  vapor. 

In  formulating  this  last  statement  we  have,  however, 
employed,  not  Herschel's  figures,  but  those  resulting 
from  later  observations,  which  increase  the  solar  radia- 
tion almost  fifty  per  cent.,  making  the  thickness  of  the 
ice-crust  which  the  sun  would  melt  off  of  his  own  sur- 
face in  a  minute  to  be  much  nearer  sixty  feet  than  forty. 

To  put  it  a  little  more  technically,  expressing  it  in 
terms  of  the  modern  scientific  units,  the  sun's  radiation 
amounts  to  more  than  1,200,000  calories  per  minute  for 
each  square  metre  of  his  surface,  the  calory*  or  heat- 
unit,  being  the  quantity  of  heat  which  will  raise  the 
temperature  of  a  kilogramme  of  water  one  degree  cen- 
tigrade. 

An  easy  calculation  shows  that,  to  produce  this 
amount  of  heat  by  combustion  would  require  the  hourly 
burning  of  a  layer  of  anthracite  coal  more  than  nineteen 
feet  (five  metres)  thick  over  the  entire  surface  of  the 
sun — nine  tenths  of  a  ton  per  hour  on  each  square  foot 
of  surface — at  least  nine  times  as  much  as  the  consump- 

*  This  is  the  engineers'  "  calory."     For  many  scientific  purposes  the 
"  small  calory,"  a  thousand  times  less,  is  more  conveniently  used — viz., 
the  amount  of  heat  which  will  raise  the  temperature  of  one  gramme  of 
water  one  centigrade  degree. 
20 


290  THE  SUN. 

tion  of  the  most  powerful  blast-furnace  known  to  art. 
It  is  equivalent  to  a  continuous  evolution  of  about  twelve 
thousand  horse-power  on  every  square  foot  of  the  sun's 
whole  area.  As  Sir  William  Thomson  (now  Lord  Kel- 
vin) has  shown,  the  sun,  if  it  were  composed  of  solid 
coal,  and  produced  its  heat  by  combustion,  would  burn 
out  in  less  than  five  thousand  years. 

Of  this  enormous  outflow  of  heat  the  earth  of  course 
intercepts  only  a  small  portion,  about  sMnnr.ihnjMnro'- 
But  even  this  minute  fraction  is  enough  to  melt  yearly, 
at  the  earth's  equator,  a  layer  of  ice  about  two  hundred 
and  twenty-five  feet  thick.  If  we  choose  to  express  it 
in  terms  of  "  power,"  we  find  that  this  is  equivalent,  for 
each  square  foot  of  surface,  to  one  hundred  and  twenty 
tons  raised  to  the  height  of  a  mile;  and,  taking  the 
whole  surface  of  the  earth,  the  average  energy  received 
from  the  sun  is  ninety-six  mile-tons  yearly,  or  one  horse- 
power continuously  acting,  to  every  sixteen  square  feet 
of  the  earth's  surface.  Most  of  this,  of  course,  is  ex- 
pended merely  in  maintaining  the  earth's  temperature ; 
but  a  small  portion,  perhaps  10100  of  the  whole,  as  esti- 
mated by  Helmholtz,  is  stored  away  by  animals  and  vege- 
tables, and  constitutes  an  abundant  revenue  of  power 
for  the  whole  human  race.* 

*  Several  experimenters  have  contrived  machines  for  the  purpose  of 
utilizing  the  solar  heat  as  a  source  of  mechanical  energy,  among  whom 
Ericsson  and  Mouchot  have  been  most  successful.  M.  Pifre  describes 
some  results  from  a  machine  of  Mouchot's  construction,  claiming  to  have 
utilized  more  than  seventy  per  cent,  of  the  heat  which  falls  on  the  mirrors 
of  the  instrument — something  over  twelve  calories  to  a  square  metre.  We 
do  not  mean,  of  course,  that  this  percentage  of  the  total  solar  energy  ap- 
peared as  mechanical  power  in  the  engine,  but  only  in  its  boiler.  The 
machine  had  a  mirror- surf  ace  of  nearly  a  hundred  square  feet,  and  gave 
not  quite  a  horse-power.  Ericsson's  engine,  exhibited  for  several  years 
jn  the  American  Institute  Fairs  in  New  York,  about  1886,  was  still  more 
efficient  and  powerful,  driving  a  two  and  a  half  horse-power  engine  vigor- 


THE   SUN'S  LIGHT  AND   HEAT. 


291 


FIG.  95. 


If  we  inquire  what  becomes  of  that  principal  portion 
of  the  solar  heat  which  misses  the  planets  and  passes  off 
into  space,  no  certain  answer  can  be  given.  Remem- 
bering, however,  that  space  is  full  of  isolated  particles 
of  matter  (which  we  encounter 
from  time  to  time  as  shooting- 
stars),  we  can  see  that  nearer  or 
more  remotely  in  its  course  each 
solar  ray  is  sure  to  reach  a  rest- 
ing-place. It  has  been  suggested 
that  the  sun  sends  heat  only  to- 
ward its  planets  ;  that  the  action 
of  radiant  heat,  like  that  of  gravi- 
tation, is  only  between  masses. 
But  scientific  investigation  so  far 
fails  to  prove  it.  The  energy 
radiated  from  a  heated  globe  is 
found  to  be  alike  in  all  direc- 
tions, and  wholly  independent  of 
the  bodies  which  receive  it,  nor 
is  there  the  slightest  reason  to 
suppose  the  sun  any  way  differ- 
ent in  this  respect  from  every 
other  incandescent  mass. 

Pouillet's  experiments  were 
made  about  the  same  time  as  HerscheFs,  but  with  a  dif- 
ferent apparatus,  though  based  on  the  same  principles. 
He  named  his  instrument  the  pyrheliometer,  or  "  meas- 
urer of  solar  fire."  Fig.  95  represents  it.  The  little 
snuffbox-like  vessel,  #,  5,  of  silver-plated  copper,  black- 
ened on  the  upper  surface,  contains  a  weighed  quantity 

ously.  It  is  quite  likely  that  such  machines  will  prove  practically  useful 
in  countries  where  sunshine  can  be  depended  on  at  certain  seasons,  as  in 
Egypt  and  California. 


292 


THE   SUN. 


of  water,  and  a  thermometer  is  immersed  in  it,  the  mer- 
cury in  its  stem  being  visible  at  d.  The  disk,  <?,  0,  makes 
it  easy  to  point  the  instrument  squarely  to  the  sun,  by 
directing  it  so  that  the  shadow  of  a  falls  concentrically 
upon  this  disk.  The  button  at  the  lower  end  is  for  the 
purpose  of  agitating  the  water  in  the  vessel  a,  a,  by  sim- 
ply turning  the  whole  thing  on  its  axis,  in  the  collar  0,  c. 
The  instrument  is  much  more  convenient  than  Herschel's 
apparatus,  but  hardly  as  accurate,  except  under  very  care- 
ful manipulation  and  protection  from  currents  of  air. 


FIG.  96. 


CEOVA'S  PTRHELIOMETEK. 


Crova  has  modified  it  by  filling  the  upper  vessel 
with  mercury.  For  relative  measurements,  as,  for  in- 
stance, a  comparison  of  the  amounts  of  heat  received 
from  the  sun  at  different  hours,  Crova  employs  a  slight- 
ly different  instrument,  represented  in  Fig.  96. 

An  exceedingly  sensitive  alcohol  thermometer,  shown 
separately  at  T^  with  a  large  bulb  carefully  blackened, 


THE   SUN'S  LIGHT   AND   HEAT.  293 

is  inclosed  in  a  double-walled  sphere,  _#,  nickel-plated 
on  the  outside.  An  opening  in  the  walls  of  the  sphere, 
carefully  aligned  with  a  similar  opening  in  a  double 
screen,  £,]  allows  a  beam  of  light  to  fall  upon  the  ther- 
mometer-bulb, the  beam  being  about  two  thirds  the 
diameter  of  the  bulb.  The  thermometer  is  constructed 
with  a  supplementary  reservoir,  /»,  at  the  lower  end,  b;y 
means  of  which  the  end  of  the  indicating  column  can 
be  made  to  fall  near  the  middle  of  the  scale  at  any  tem- 
perature, the  object  being  to  measure  only  changes  of 
temperature,  not  absolute  temperatures.  The  bulb  and 
tube  are  so  proportioned  that  a  degree  on  the  scale  is 
nearly  half  an  inch  long,  thus  permitting  great  accuracy 
of  reading.  In  order,  however,  to  determine  just  how 
much  heat  is  required  to  raise  the  thermometer  of  this 
instrument  1°,  it  is  necessary  to  compare  it  with  one  of 
the  standard  instruments,  by  exposing  it  to  the  sun  at 
the  same  time. 

This  method  of  procedure,  by  which  we  determine 
the  rate  at  which  a  sunbeam  of  given  dimensions  com- 
municates heat  to  a  measured  mass  of  matter,  is  known 
as  the  dynamic  method.  It  is  somewhat  inconvenient 
in  requiring  considerable  time  and  a  number  of  readings. 

There  is  a  different  process  for  deducing  the  same 
results,  which  has  been  employed  by  Waterston,  Erics- 
son, Secchi,  Yiolle,  and  others,  and  may  be  called  the 
statical  method.  It  consists  essentially  in  observing 
how  much  the  sun  will  raise  the  temperature  of  a  body 
exposed  to  its  rays  above  that  of  the  inclosure  in  which 
it  is  placed,  this  inclosure  being  kept  at  a  fixed  and 
known  temperature  by  the  circulation  of  water,  or  some 
such  means.  Instruments  based  on  this  principle  are 
called  actinometers.  Of  these,  probably  the  most  com- 
plete in  its  arrangements  is  that  of  Yiolle,  described  in 


294 


THE  SUN. 


his  paper  upon  the  mean  temperature  of  the  sun's  sur- 
face, published  in  the  "  Annales  de  Chimie,"  in  1877. 
We  give  a  diagram  of  the  instrument.  It  consists  of 
two  concentric  spheres  of  thin  metal,  the  outer  twenty- 
three  centimetres  in  diameter,  the  inner  fifteen  centi- 
metres. The  outer  is  polished  on  the  outside ;  the 

Fio.  97. 


VIOLLE'S  ACTINOMBTKK. 

inner  is  blackened  on  the  inside.  The  space  between 
the  two  spheres  is  filled  with  water,  which  is  kept  at  a 
uniform  temperature  either  by  mixing  snow  or  ice  with 
it,  or  else  by  a  current  circulated  through  it  by  means 
of  the  stopcocks  t,  t.  A  sensitive  thermometer,  Ty  has 
its  blackened  bulb  placed  in  the  center  of  the  inner 


THE  SUN'S  LIGHT  AND  HEAT.  295 

sphere,  the  stem  reaching  outside  through  a  tubulure 
provided  for  the  purpose.  Two  opposite  openings, 
shown  in  the  figure,  allow  a  beam  of  sunlight  to  pass 
through  the  globes.  A  perforated  screen  at  D  limits 
its  diameter,  so  that  none  of  it  shall  touch  the  walls 
of  the  vessel,  though  the  thermometer-bulb  is  entirely 
covered  by  it.  A  small  screen  at  M  allows  the  observer 
to  see  the  shadow  of  the  thermometer-bulb,  and  so  to 
perceive  whether  the  tube  through  which  the  light 
enters  is  properly  directed.  If  the  apparatus  is  mount- 
ed upon  what  is  called  an  equatorial  stand,  like  a  tele- 
scope, and  provided  with  clock-work,  the  whole  labor 
of  observation  will  consist  merely  in  reading  the  ther- 
mometer. The  difference  between  its  temperature  and 
that  of  the  water  in  the  surrounding  shell  gives  the 
necessary  data  for  calculating  the  intensity  of  the  solar 
radiation  at  the  time  of  reading,  since  the  heat  received 
by  the  thermometer  from  the  sun  and  shell  together 
must  just  equal  that  radiated  back  by  the  thermometer- 
bulb  to  the  shell,  after  allowing  for  the  orifices. 

Yiolle  found  that  at  noon  on  a  fair  day  the  ther- 
mometer of  this  apparatus  generally  stood,  when  ex- 
posed to  the  sun,  from  10*5°  to  12*5°  centigrade  (i.  e., 
18-9°  to  22-5°  Fahr.)  above  the  temperature  of  the  shell 
when  the  latter  was  filled  with  ice-water.  If  it  was 
filled  with  boiling  water,  as  in  some  of  his  experiments, 
the  difference  became  less  by  about  1°  centigrade. 

The  results  obtained  with  instruments  of  this  class, 
of  course,  agree  very  closely  with  those  reached  by  the 
dynamic  method. 

Instead  of  stating  how  much  ice  would  be  melted 
in  a  minute  by  a  given  sunbeam,  we  may  give  the  num- 
ber of  calories  received  in  one  minute  by  a  square  metre 
of  surface  exposed  perpendicularly  to  the  sun's  rays  at 


296  THE  SUN- 

the  upper  surface  of  the  atmosphere.  This  number, 
which  may  be  taken  as  the  measure  of  the  sun's  radia- 
tion, is  called  u  the  solar  constant"  and,  according  to 
different  experimenters,  ranges  from  Pouillet's  estimate, 
1T'6,  to  that  of  Langley,  which  is  30'0,  the  latest  and 
most  reliable.  Forbes  found  28'2,  and  Crova  and  Violle, 
at  a  later  date,  23*2  and  25 '4  respectively.  In  the  pre- 
ceding editions  of  this  book  it  was  taken  at  25,  but 
there  is  now  no  question  that  Langley's  result  should  be 
substituted,  since  he  has  discovered  an  important  error 
in  the  work  of  his  predecessors,  and  by  his  laborious 
course  of  "  bolometric  "  observations  (to  be  discussed  a 
little  later)  has  supplied  the  data  for  the  necessary  cor- 
rection. 

Instead  of  stating  the  solar  constant  as  thirty  engi- 
neering calories  per  square  metre  per  minute,  some 
prefer  to  state  it  as  three  small  calorics  per  square  cen~ 
timetre  per  minute,  which  comes  to  the  same  thing : 
the  unit  of  heat  is  a  thousand  times  smaller,  and  the 
unit  of  surface  ten  thousand  times  less  than  in  the  first 
statement.  Professor  Langley  himself  prefers  this  lat- 
ter form.  Those  who  insist  on  expressing  all  scientific 
measures  in  the  so-called  "  C.  G.  S.  system,"  give  the 
solar  constant  as  0'05  small  calories  per  square  centi- 
metre per  second,  which,  of  course,  is  equivalent  to 
either  of  the  other  forms. 

It  is  not  yet  by  any  means  certain  that  this  "  solar 
constant "  is  really  constant :  indeed,  it  is  quite  certain 
that  it  is  not  strictly  so — that  the  amount  of  heat  radi- 
ated by  the  sun  must  vary  more  or  less  with  the  changes 
which  we  know  to  occur  upon  its  surface ;  at  the  same 
time  there  is  no  reason  to  suppose  that  the  variations  are 
very  great.  It  is,  however,  one  of  the  most  important 
and  difficult  problems  of  solar  physics  now  pending  to 


THE  SUN'S  LIGHT  AND  HEAT.  297 

determine  the  actual  amount  of  these  variations,  and  to 
ascertain  the  laws  that  govern  them. 

By  far  the  most  difficult  part  of  the  experimental 
problem  of  the  solar  constant  lies  in  the  determination 
of  the  large  and  troublesome  corrections  to  be  applied 
on  account  of  the  absorption  of  the  earth's  atmosphere. 
It  would  take  us  too  far  to  discuss  the  formulae  and  meth- 
ods of  calculation  which  have  been  proposed.  They  are 
necessarily  very  complicated  (those,  at  any  rate,  which 
are  tolerably  accurate  in  their  results),  because  they  have 
to  take  into  account  the  meteorological  conditions,  espe- 
cially the  hygrometric  state  of  the  air.  Besides  this, 
the  absorption  varies  greatly  for  radiations  of  different 
pitch,  so  that  the  violet  rays,  which  are  photographically 
the  most  active,  suffer  more  than  the  green  and  yellow, 
which  are  most  effective  in  the  growth  of  plants ;  and 
these  more  than  the  red;  and  the  red,  in  their  turn, 
much  more  than  the  low-pitched,  slowly  vibrating  waves 
which,  though  invisible,  are  still  powerful  carriers  of 
energy. 

Speaking  loosely,  it  may  be  estimated  that,  at  the 
sea-level,  in  fair  weather,  neither  excessively  moist  nor 
dry,  about  thirty  per  cent,  of  the  solar  radiation  is 
absorbed  when  the  sun  is  at  the  zenith,  and  at  least 
seventy-five  per  cent,  at  the  horizon.  Of  the  rays 
striking  the  upper  surface  of  the  atmosphere,  between 
forty-five  and  fifty  per  cent.,  therefore,  are  generally 
intercepted  in  the  air,  even  when  there  are  no  clouds. 

Of  course,  it  does  not  follow  that  the  heat  absorbed 
in  our  atmosphere  is  lost  to  the  earth.  Far  from  it : 
the  air  itself  becomes  warmed  and  communicates  its 
heat  to  the  earth ;  and,  since  the  atmosphere  intercepts 
a  large  proportion  of  the  heat  which  the  earth  would 
radiate  into  space  if  not  thus  blanketed,  the  temperature 


298  THE 

of  the  earth  is  kept  much  higher  than  it  would  be  if 
there  were  no  air. 

The  earlier  investigators  sought  to  determine  the 
amount  of  the  necessary  correction  for  this  atmospheric 
absorption  "  in  the  gross,"  so  to  speak.  That  is,  they 
ascertained  by  their  experiments  the  whole  amount  of 
heat  received  from  the  sun  at  different  apparent  eleva- 
tions, when  its  rays  had  to  penetrate  different  thick- 
nesses of  air ;  and  from  these  results  they  attempted  to 
deduce  the  amount  of  heat  that  would  have  been  re- 
ceived if  no  air  had  intervened.  In  making  the  calcu- 
lation they  employed  a  well-known  formula  which  gives 
with  practical  correctness,  for  a  homogeneous  ray  (all  of 
one  wave-length)  of  light  or  heat,  the  amount  trans- 
mitted through  any  given  thickness  of  an  absorbing 
medium  when  we  have  once  for  all  determined  the  per- 
centage  transmitted  through  a  stratum  of  the  medium 
one  unit  thick.  This  percentage  is  called  "  the  coeffi- 
cient of  transmission,"  and  can  be  found  by  measuring 
the  amount  of  this  homogeneous  light  or  heat  trans- 
mitted through  any  two  strata  that  differ  considerably 
in  thickness. 

Now  the  experimenters  knew  perfectly  well  that 
the  radiant  heat  they  were  dealing  with  was  not  homo- 
geneous, but  composed  of  rays  of  widely  different  wave- 
length :  they  supposed,  however,  that  by  treating  the 
matter  as  they  did  they  would  get  a  sort  of  average 
coefficient  of  transmission  which  would  be  practically 
correct. 

In  this  they  were  greatly  mistaken.  Professor  Lang- 
ley  was  the  first  to  point  out  the  error,  to  show  that  its 
correction  would  largely  increase  the  estimate  of  the 
solar  constant,  and  to  invent  the  apparatus  and  make 
the  observations  necessary  to  bring  out  the  truth.  He 


THE  SUN'S  LIGHT  AXD  HEAT. 

saw  that  it  was  indispensable  to  determine  the  atmos- 
pheric coefficient  of  transmission  separately  for  each  of 
a  multitude  of  rays  of  different  wave-length  distributed 
all  along  the  spectrum — the  invisible  portions  as  well  as 
the  visible — and  to  determine  also  for  each  ray  its  pro- 
portion contributed  to  the  total  amount  of  sunlight 
energy.  For  this  purpose  he  was  obliged  to  devise  a 
heat-measurer  far  more  delicate  than  any  before  in  use, 
and  with  it  to  explore  the  spectrum  from  end  to  end, 
both  from  stations  near  the  level  of  the  sea  and  from 
the  top  of  a  lofty  mountain  (Mount  Whitney,  15,000 
feet  high). 

The  new  heat-measurer,  which  he  called  the  "  bolom- 
eter," depends  upon  the  principle,  long  known  and 
previously  applied  by  Jamin  and  others,  that  the  elec- 
trical resistance  of  a  metal  is  increased  by  warming  it. 
The  sensitive  "  nerve,"  if  we  may  call  it  so,  is  a  little 
strip  of  iron  or  palladium  about  a  third  of  an  inch  long, 
by  -g-J-g-  wide,  and  16^00  thick.  This  is  balanced  against 
a  similar  strip  placed  near  the  first,  but  screened  from 
the  heat  rays  to  be  measured. 

The  two  strips  form  the  so-called  "  arms  "  of  an  elec- 
tric balance,  and  are  connected  with  a  delicate  gal- 
vanometer, the  index  of  which  (a  spot  of  light)  moves 
whenever  there  is  any  difference  of  temperature  be- 
tween them.  The  instrument  with  which  Langley  .has 
lately  been  making  his  wonderful  map  of  the  invisible 
regions  of  the  spectrum,  indicates  distinctly  one  mil- 
lionth of  a  centigrade  degree. 

The  strips  are  mounted  in  a  little  tube  of  hard  rub- 
ber, so  as  to  be  carefully  protected  from  all  outside 
influences,  except  that  a  narrow  slit  in  front  of  the 
"  nerve "  leaves  it  free  to  receive  the  rays  to  be  ob- 
served. 


300 


THE  SUN. 


The  bolometer  is  used  in  connection  with  a  large 
spectroscope,  taking  the  place  of  the  eyepiece :  in  this 
"  spectro-bolometer,"  as  Langley  calls  the  combination, 
the  prism,  if  one  is  used,  must  be  of  rock-salt,  the  only 
substance,  so  far  as  known,  which  freely  transmits  the 
invisible  rays  of  the  heat  spectrum.  The  lenses  of  the 
spectroscope  are  also  of  the  same  material. 

For  some  purposes  a  grating  can  be  used,  but  usually 
its  spectrum  is  too  feeble. 

The  light  is  brought  to  the  collimator  slit  by  a  mir- 
ror, and  as  the  grating  or  prism  is  turned  the  spectrum 


FIG.  98. 


LAXGLEY'S  SPECTBO- BOLOMETER,  AS  USED  FOE  MAPPING  THE  ENERGY  OF  THE 
PRISMATIC  SPECTRUM. 

traverses  the  bolometer  slit,  and  the  galvanometer  index 
by  its  motion  indicates  the  dark  lines  and  bright  inter- 
spaces as  they  pass  it  in  procession. 


THE   SUN'S  LIGHT  AND   HEAT.  301 

Fig.  98  represents  one  form  of  the  instrument.  The 
rays  arrive  through  the  lens  L,  pass  through  the  prism 
P,  and  are  then  reflected  back  from  the  mirror  M  to 
the  bolometer  at  B,  from  which  the  wires  go  to  the 
galvanometer  and  to  the  battery  which  furnishes  the 
current. 

Until  quite  recently  the  galvanometer  readings  had 
to  be  made  by  the  eye  and  the  records  by  hand,  an  ex- 
ceedingly tedious  business,  but  very  lately  the  apparatus 
has  been  made  automatic.  The  galvanometer  index  (a 
spot  of  light)  falls  upon  a  sensitive  plate  which  is  car- 
ried along  in  precise  correspondence  with  the  motion  of 
the  grating  or  prism.  The  result  is  an  irregular  curve 
on  the  developed  plate,  in  which  dark  lines  of  the  spec- 
trum are  represented  by  notches. 

In  this  way  work  that  would  have  taken  months  by 
the  old  methods  can  be  done  in  a  single  afternoon.  By 
a  simple  process,  aLso  automatic,  the  curve  can  be  trans- 
formed into  a  picture  of  the  spectrum  showing  its  dark 
lines  and  other  characteristics  just  as  does  a  photograph. 

In  1894  Langley  published  the  map  of  the  invisible 
spectrum,  which  we  give  as  Fig.  99.  The  reader  will 


THE  INFRA-RED  SPECTBTTM  —  LANGLBT. 


notice  how  very  short  is  the  visible  portion  of  the  spec- 
trum, as  compared  with  the  extensive  range  of  rays  of 
longer  wave-length.  The  amount  of  energy,  however, 
contained  in  the  portion  of  the  spectrum  below  (to  the 
fight  of)  the  point  marked  3  is  extremely  small,  though 


302 


THE  SUN. 


still  sensible  to  a  point  far  beyond  the  limit  of  the  fig- 
ure. To  a  certain  extent  some  of  the  salient  peculiari- 
ties of  the  upper  part  of  this  invisible  spectrum,  as  far 
as  X  on  the  map,  had  been  brought  to  light  by  earlier 
investigators,  especially  Becquerel,  Lamansky,  and  Ab- 
ney,  the  last  of  whom  even  succeeded  in  photographing 
a  part  of  it.  But  Langley  was  the  first  to  give  us  any- 
thing that  could  be  called  a  map ;  and  in  some  portions 
his  large-scale  map,  which  we  can  not  well  reproduce 
here,  is  already  fairly  comparable  in  detail  and  accuracy 
with  Kirchhoff's  map  of  the  visible  spectrum. 

Experiments  with  the  thermopile  show  that  the  heat 
radiated  by  the  solar  disk  varies,  like  the  light,  very 
considerably  from  the  center  to  the  edges.  The  first 
observations  of  this  kind  were  made  by  Professor  Henry 
at  Princeton  in  1845,  and  have  since  been  repeated  by 
many  others,  Secchi  and  Langley  especially.  Accord- 
ing to  Langley,  the  heat  emitted  from  a  point  about  20" 
from  the  limb  is  only  one  half  that  from  the  same  extent 
of  surface  at  the  center  of  the  disk. 


Distance  from  center. 
Kadiua  =  1-00. 

HEAT  RADIATION. 

Langley. 

Frost. 

Wilson. 

o-oo 

100 

100-0 

100-0 

O'lO 

99-9 

99-8 

0-20 

... 

99-4 

99-5 

0-25 

99 

.... 

99-3 

0-30 

98-4 

98-9 

0-40 

98-0 

97-2 

0-50 

*95 

93-6 

95-3 

0'60 

89-8 

92-2 

0'70 

84-6 

87'8 

0-76 

'86 

t 

85'3 

0-80 

77:9 

82-5 

0-90 

.  . 

68-0 

72-0 

0-95 

62 

.... 

61-3 

0-98 

50 

61-5 

1-00 

... 

'(39) 

42-9 

THE  SUN'S  LIGHT  AND   HEAT.  3Q3 

More  recently  Frost  at  Potsdam  and  Wilson  at 
Darainona  have  reinvestigated  the  subject  a  little  more 
fully,  and  in  the  preceding  table  we  give  their  results 
as  well  as  Langley 's. 

If  we  compare  this  table  with  that  given  on  page 
281,  which  gives  the  variation  of  luminosity  from  cen- 
ter to  edge  of  the  solar  disk,  it  is  at  once  evident,  as 
Langley  was  the  first  to  point  out,  in  1875,  that  the 
absorption  is,  to  a  certain  extent,  selective,  the  short 
waves  of  the  solar  radiation  being  more  affected  than 
the  long.  Besides  this  regular  variation  of  the  radiation 
from  center  to  edge,  Secchi,  in  1852,  found,  or  thought 
he  found,  a  notable  difference  between  the  radiation 
from  the  equator  of  the  sun  and  that  from  the  higher 
latitudes,  the  difference  being  at  least  one  sixteenth 
between  the  equator  and  latitude  30°.  The  northern 
hemisphere  he  also  found  to  be  a  little  hotter  than  the 
southern.  Later  investigators  (Langley  especially)  have 
failed  to  find  any  such  difference ;  and  on  the  whole  it 
seems  probable  that  Secchi  was  mistaken,  though  this 
is  not  certain,  as  it  would  be  quite  unsafe  to  assert  that 
the  actual  condition  of  the  sun's  surface  may  not  have 
changed  between  1852  and  1876. 

In  connection  with  the  absorption  of  the  solar  at- 
mosphere, Langley  has  ventured  some  interesting  specu- 
lations. After  showing  that  variations  in  the  number 
and  magnitude  of  sun-spots  can  not  directly  produce 
any  sensible  effect  upon  terrestrial  temperatures,  he 
calls  attention  to  the  fact  that  even  slight  changes  in 
the  depth  and  density  of  the  sun's  absorbing  layer  would 
make  a  great  difference ;  and  he  raises  the  question 
whether  we  may  not  find  here  the  explanation  of  glacial 
and  carboniferous  periods  in  the  earth's  history.  It  is 
quite  certain  that,  were  the  envelope  removed,  the  solar 


304  THE  SUN. 

radiation  would  be  about  doubled,  and  perhaps  increased 
in  a  much  higher  ratio,  while  any  considerable  increase 
of  its  thickness  would  so  diminish  our  heat-supply  as 
to  give  us  perpetual  winter. 

As  yet  our  means  of  observation  have  not  sufficed 
to  detect  with  certainty  any  variations  in  the  amount  of 
heat  emitted  by  the  sun  at  different  times.  That  there 
are  such  variations  is  almost  certain,  since  the  nuclei  ol 
sun-spots  radiate  much  less  heat,  as  well  as  light,  than 
neighboring  regions  of  the  solar  surface,  and  the  faculse 
more :  this  has  been  directly  determined  with  the  ther- 
mopile. 

Some  very  ingenious  instruments  have  of  late  years 
been  devised  and  constructed  by  the  younger  Angstrom 
for  the  purpose  of  recording  automatically  the  inten- 
sity of  solar  radiation  during  an  entire  day,  and  sum- 
ming up  the  total  amount — a  process  which,  if  carried 
out  every  clear  day  for  several  years  at  some  suitable 
station,  ought,  after  a  thorough  discussion,  to  furnish 
interesting  data  as  to  the  thermal  activity  of  the  sun. 
The  difficulties,  however,  depending  on  the  continual 
variations  in  the  meteorological  conditions  are  enor- 
mous; the  truly  solar  variations  seem  to  be  utterly 
masked  and  overpowered  by  those  that  originate  in  our 
own  atmosphere. 

As  was  said  in  the  chapter  upon  the  sun-spots,  we 
are  as  yet  entirely  uncertain  whether,  at  the  time  of  a 
sun-spot  maximum,  the  solar  radiation  is  more  or  less 
powerful  than  the  average. 

There  has  been  a  great  deal  of  pretty  vigorous  dis- 
cussion as  to  the  temperature  of  the  sun,  and  that  the 
subject  is  a  difficult  one  is  evident  enough  from  the 
wide  discrepancy  between  the  estimates  of  the  highest 


THE  SUN'S  LIGHT  AND   HEAT.  305 

authorities.  For  instance,  Secchi  originally  contended 
for  a  temperature  of  about  18,000,000°  Fahr.  (though 
he  afterward  lowered  his  estimate  to  about  250,000°) ; 
Ericsson  puts  the  figure  at  4,000,000°  or  5,000,000°; 
Zollner,  Spoerer,  and  Lane  name  temperatures  ranging 
from  50,000°  to  100,000°  Fahr.,  while  Pouillet,  Vicaire, 
and  Deville  have  put  it  as  low  as  between  3,000°  and 
10,000°  Fahr. 

The  difficulty  is  twofold.  In  the  first  place,  the  sun 
can  not  properly  be  said  to  have  a  temperature  any 
more  than  the  earth's  atmosphere  can.  The  tempera- 
ture of  different  portions  of  the  solar  envelope  must 
vary  enormously,  increasing  fast  as  we  descend  below 
or  ascend  above  the  surface.  There  may  be  a  differ- 
ence of  thousands  of  degrees  between  the  temperatures 
of  the  chromosphere  and  of  the  photosphere ;  and  still 
other  thousands  between  that  of  the  photosphere  and 
the  depths  beneath. 

We  may,  however,  partially  evade  this  difficulty  by 
substituting  as  the  object  of  inquiry  the  sun's  effective 
temperature — i.  e.,  instead  of  seeking  to  ascertain  the 
actual  temperature  of  different  parts  of  the  sun's  sur- 
face, we  may  inquire  what  temperature  would  have  to 
be  given  to  a  uniform  surface  of  standard  radiating 
power  (a  surface  covered  with  lampblack  is  generally 
taken  as  this  standard),  and  of  the  same  size  as  the  sun, 
in  order  that  it  might  emit  as  much  heat  as  the  sun 
actually  does.  In  this  way  we  obtain  a  perfectly  defi- 
nite object  of  investigation.  But  the  problem  still  re- 
mains very  difficult,  and  has  obtained  as  yet  no  entirely 
satisfactory  solution.  The  difficulty  lies  in  our  ignor' 
ance  as  to  the  laws  which  connect  the  temperature  of  a 
surface  with  the  amount  of  heat  radiated  per  second. 
So  long  as  the  temperature  of  the  radiating  body  does 
21 


306  THE  SUN. 

not  greatly  exceed  that  of  surrounding  space,  the  heat 
emitted  is  very  nearly  proportional  to  the  excess  of 
temperature.  The  extremely  high  values  of  the  solar 
temperature  asserted  by  Secchi  and  Ericsson  depend 
upon  the  assumption  of  this  law  (known  as  Newton's) 
of  proportionality  between  the  heat  radiated  and  the 
temperature  of  the  radiating  mass — a  law  which  direct 
experiment  proves  to  be  untrue  as  soon  as  the  tempera- 
ture rises  a  little.  In  reality,  the  amount  of  heat  radi- 
ated increases  much  faster  than  the  temperature. 

More  than  fifty  years  ago  the  French  physicists, 
Dulong  and  Petit,  by  a  series  of  elaborate  experiments, 
deduced  an  empirical  formula,  which  answered  pretty 
satisfactorily  for  temperatures  up  to  a  dull-red  heat. 
By  applying  this  formula,  Pouillet,  Yicaire,  and  others 
arrived  at  the  low  solar  temperatures  assigned  by  them. 
It  is,  however,  evidently  unsafe  to  apply  a  purely  em- 
pirical formula  to  circumstances  so  far  outside  the  range 
of  the  observations  upon  which  it  was  founded,  and,  in 
fact,  within  a  few  years  several  experimenters,  Rosetti 
especially,  have  shown  that  it  needs  modification,  even 
in  the  investigation  of  artificial  temperatures  like  that 
of  the  electric  arc.  Rosetti,  from  his  observations,  has 
deduced  a  different  law  of  radiation,  and  by  its  appli- 
cation finds  10,000°  Cent.,  or  18,000°  Fahr.,  as  the  effec- 
tive temperature  of  the  sun — a  result  which,  all  things 
considered,  seems  to  the  writer  more  reasonable  and  bet- 
ter founded  than  any  of  the  earlier  estimates.  Rosetti 
considers  that  this  is  also  pretty  nearly  the  actual  tem- 
perature of  the  upper  layers  of  the  photosphere.  The 
radiating  power  of  the  photospheric  clouds,  to  be  sure, 
can  hardly  be  as  great  as  that  of  lampblack ;  but,  on  the 
other  hand,  their  radiation  is  supplemented  by  that  of 
other  layers,  both  above  and  below. 


THE  SUN'S  LIGHT  AND  HEAT.  307 

Still  more  recently  (in  1892)  Le  Chatelier  has  de- 
duced an  effective  temperature  of  7,600°  Cent,  (or  about 
13,700  Fahr.)  by  a  study  of  the  intensity  of  certain  red 
rays  of  the  sun  compared  with  the  intensity  of  the  same 
rays  in  the  radiations  of  certain  bodies  heated  to  lumi- 
nosity. 

Still  later  Wilson  and  Gray,  by  a  very  elaborate  in- 
vestigation, which,  on  the  whole,  seems  entitled  to  take 
the  precedence  over  all  competitors  so  far,  have  obtained 
as  a  result  8,000°  *  Cent.,  or  14,400°  Fahr.  Their  appa- 
ratus and  methods  were  different  from  any  previously 
employed  :  as  a  measurer  of  the  radiation  they  used  the 
"  radio-micrometer  "  of  Boys,  which  combines  thermo- 
pile and  galvanometer  in  one,  and  possibly  surpasses  in 
sensitiveness  even  the  "bolometer"  of  Langley.  For 
the  determination  of  temperatures,  by  which  to  inter- 
pret the  radiation  measures,  they  used  a  modified  form 
of  Joly's  meldometer,  in  which  a  strip  of  platinum  is 
raised  to  any  desired  temperature  short  of  melting  by 
a  current  from  a  storage  battery  or  dynamo.  The  ap- 
paratus utilizes  the  inventions  and  resources  of  the  new- 
est scientific  art,  and  would  have  been  impossible  a 
dozen  years  ago. 

It  is  worth  noting  that  their  observations  disprove 
Dulong  and  Petit's  law  of  radiation,  and  seem  to  con- 
firm the  much  simpler  one  proposed  by  Stephan  of 
Vienna,  and  theoretically  deduced  by  Boltzmann  as  a 
consequence  of  the  electro-magnetic  theory  of  light. 
The  formula  is  simply  this,  R  =  a  X  T4 :  in  which  R  is 
the  intensity  of  the  radiation  in  units  of  energy,  a  is  a 
constant  coefficient,  depending  on  the  nature  of  the  body 

*  The  highest  artificial  temperatures  are  supposed  to  fall  short  of 
2,500°  Cent.  Possibly  this  figure  may  now  be  exceeded  in  some  of  the 
electric  furnaces. 


308  THE  SUN. 

and  units  of  energy  and  temperature  employed,  while 
T  is  the  temperature  of  the  body  above  the  absolute 
zero,  and  is  equal  to  the  centigrade  temperature  with 
273°  added. 

If  the  effective  temperature  of  the  sun  were  doubled 
we  should  receive  sixteen  times  as  much  heat,  and  ten 
thousand  times  as  much  if  its  temperature  were  in- 
creased only  ten  times. 

Scheiner,  of  Potsdam,  has  shown  spectroscopically 
that  the  temperature  of  the  sun's  reversing  layer  is  al- 
most certainly  intermediate  between  that  of  the  electric 
arc  and  the  much  higher  temperature  of  the  Leyden-jar 
spark.  In  the  spectrum  of  magnesium  there  are  two 
lines  at  \  4,482  and  X  4,352  which  are  strikingly  con- 
trasted in  behavior.  The  former  is  strong  in  the  spark- 
spectrum  but  hardly  visible  in  that  of  the  arc,  while  with 
4,352  the  reverse  is  true ;  and  Scheiner  shows  that  the 
difference  depends  upon  temperature.  ISTow  in  the  solar- 
spectrum  4,352  is  a  conspicuous  dark  line,  and  the  other 
is  missing ;  whence  the  inference  that  the  magnesium 
vapor  in  the  reversing  layer  is  about  as  hot  as  the  electric 
arc,  and  the  photosphere  below,  of  course,  still  hotter. 

Besides  the  data  as  to  the  intensity  of  the  solar 
temperature  obtained  by  the  methods  above  mentioned, 
we  have  also  direct  evidence  of  a  very  impressive  sort. 
When  heat  is  concentrated  by  a  burning-glass,  the  tem- 
perature at  the  focus  can  not  rise  above  that  of  the 
source  of  heat,  the  effect  of  the  lens  being  simply  to 
move  the  object  at  the  focus  virtually  toward  the  sun ; 
so  that,  if  we  neglect  the  loss  of  heat  by  transmission 
through  the  glass,  the  temperature  at  the  focus  should 
be  the  same  as  that  of  a  point  placed  at  such  a  distance 
from  the  sun  that  the  solar  disk  would  seem  just  as 
large  as  the  lens  itself  viewed  from  its  own  focus. 


THE  SUN'S  LIGHT  AND  HEAT.  309 

The  most  powerful  lens  yet  constructed  thus  virtu- 
ally transports  an  object  at  its  focus  to  within  about 
two  hundred  and  fifty  thousand  miles  of  the  sun's  sur- 
face, and  in  this  focus  the  most  refractory  substances — 
platinum,  fire-clay,  the  diamond  itself — are  either  in- 
stantly melted  or  dissipated  in  vapor.  There  can  be  no 
doubt  that,  if  the  sun  were  to  come  as  near  us  as  the 
moon,  the  solid  earth  would  melt  like  wax. 

We  have  spoken,  a  few  pages  back,  of  Professor 
Langley's  experimental  comparison  between  the  brill- 
iance of  the  solar  surface  and  that  of  the  metal  in  a 
Bessemer  converter.  At  the  same  time  he  made  meas- 
urements of  the  heat  by  means  of  a  thermopile,  and 
found  the  heat  radiation  of  the  solar  surface  to  be  more 
tha/n  eighty-seven  times  as  intense  as  that  from  the  sur- 
face of  the  molten  metal.  It  will  be  recalled  that  the 
experiment  only  sets  a  lower  limit  to  the  solar  radiation, 
so  that  it  is  altogether  probable  that,  were  all  the  neces- 
sary corrections  determined  and  applied,  the  ratio  would 
be  increased  from  eighty-seven  to  at  least  a  hundred, 
and  perhaps  to  a  hundred  and  fifty.  Ericsson,  in  1872, 
made  a  somewhat  similar  comparison  in  a  different  and 
exceedingly  ingenious  manner.  He  floated  a  calorime- 
ter containing  about  ten  pounds  of  water  upon  the  sur- 
face of  a  large  mass  of  molten  iron,  by  means  of  a  raft 
of  fire-brick.  The  calorimeter  was  raised  a  little  above 
the  surface,  and  the  water  contained  was  kept  in  circu- 
lation by  suitable  mechanism.  He  found  that  the  radia- 
tion of  the  metal  was  a  trifle  over  two  hundred  and  fifty 
calories  per  minute  for  each  square  foot  of  surface. 
This  is  equivalent  to  twenty-seven  hundred  and  ninety 
calories  to  the  square  metre,  and  is  only  ^J-g-  of  the  sun's 
emission.  He  estimated  the  temperature  of  the  metal 
at  3,000°  Fahr.,  or  1,538°  Cent.  Professor  Langley,  in 


310  THE  SUN. 

his  experiment,  estimated  the  temperature  of  the  Bes- 
semer metal  much  higher — superior,  in  fact,  to  the  tem- 
perature of  melting  platinum,  which  is  usually  consid- 
ered to  be  about  2,000°  Cent.  He  bases  this  conclusion 
upon  the  fact  that  platinum  wire,  stretched  above  the 
mouth  of  the  converter,  or  dipped  into  the  issuing 
stream,  was  immediately  melted.  Since,  however,  iron 
and  its  vapor  attack  platinum  much  in  the  same  way  as 
mercury  and  its  vapor  attack  gold,  there  may  be  some 
doubt  as  to  the  correctness  of  his  estimate.  The  same 
conclusions  as  to  the  intensity  of  the  solar  temperature 
follow  from  investigations  by  Soret  and  others  as  to 
the  penetrating  power  of  the  sun's  rays,  and  from  a 
comparison  with  artificial  sources  of  heat  in  respect  to 
the  relative  proportion  of  the  rays  of  different  wave- 
lengths in  the  total  radiation.  A  body  of  low  tempera- 
ture emits  an  enormous  proportion  of  slow-swinging, 
invisible  vibrations,  while,  as  the  temperature  rises,  the 
shorter  waves  become  proportionally  more  and  more 
abundant.  Thus,  in  the  composition  of  a  body's  radia- 
tion, we  get  some  clew  to  its  temperature.  Hitherto 
all  such  tests  concur  in  putting  the  sun's  temperature 
high  above  that  of  any  known  terrestrial  flame. 

And  now  we  come  to  questions  like  these :  How  is 
such  a  heat  maintained?  How  long  has  it  lasted  al- 
ready? How  long  will  it  continue?  Are  there  any 
signs  of  either  increase  or  diminution? — questions  to 
which,  in  the  present  state  of  science,  only  somewhat 
vague  and  unsatisfactory  replies  are  possible. 

As  to  progressive  changes  in  the  amount  of  the  solar 
heat  it  can  be  said,  however,  that  there  is  no  evidence 
of  anything  of  the  sort  since  the  beginning  of  authentic 
records.  There  have  been  no  such  changes  in  the  dis- 
tribution of  plants  and  animals  within  the  last  two  thou- 


THE  SUN'S  LIGHT  AND  HEAT.  3H 

sand  years,  as  must  have  occurred  if  there  had  been, 
within  this  period,  any  appreciable  alteration  in  the 
heat  received  from  the  sun.  So  far  as  can  be  made 
out,  with  few  and  slight  exceptions,  the  vine  and  olive 
grow  just  where  they  did  in  classic  days,  and  the  same 
is  true  of  the  cereals  and  the  forest-trees.  In  the  re- 
moter past  there  have  been  undoubtedly  great  changes 
in  the  earth's  temperature,  evidenced  by  geological 
records — carboniferous  epochs,  when  the  temperature 
was  tropical  in  almost  arctic  latitudes,  and  glacial  pe- 
riods, when  our  now  temperate  zones  were  incased  in 
sheets  of  solid  ice,  as  northern  Greenland  is  at  present. 
Even  as  to  these  changes,  however,  it  is  not  yet  certain 
whether  they  are  to  be  traced  to  variations  in  the  amount 
of  heat  emitted  by  the  sun,  or  to  changes  in  the  earth 
herself,  or  in  her  orbit.  So  far  as  observation  goes,  we 
can  only  say  that  the  outpouring  of  the  solar  heat,  amaz- 
ing as  it  is,  appears  to  have  gone  on  unchanged  through 
all  the  centuries  of  human  history. 

What,  then,  maintains  the  fire  ?  It  is  quite  certain, 
in  the  first  place,  that  it  is  not  a  case  of  mere  combus- 
tion. As  has  been  said,  only  a  few  pages  back,  it  has 
been  shown  that,  even  if  the  sun  were  made  of  solid 
coal,  burning  in  pure  oxygen,  it  could  only  last  about 
six  thousand  years :  it  would  have  been  nearly  one  third 
consumed  since  the  beginning  of  the  Christian  era. 
Nor  can  the  source  of  its  heat  lie  simply  in  the  cooling 
of  its  incandescent  mass.  Huge  as  it  is,  its  temperature 
must  have  fallen  more  than  perceptibly  within  a  thou- 
sand years  if  this  were  the  case. 

Many  different  theories  have  been  proposed,  two  of 
which  now  chiefly  occupy  the  field.  One  of  them  finds 
the  chief  source  of  the  solar  heat  in  the  impact  of 
meteoric  matter,  the  other  in  the  slow  contraction  of  the 


312  THE   SUN. 

sun.  As  to  the  first,  it  is  quite  certain  that  a  part  of 
the  solar  heat  is  produced  in  that  way  ;  but  the  question 
is  whether  the  supply  of  meteoric  matter  is  sufficient 
to  account  for  any  great  proportion  of  the  whole.  As 
to  the  second,  on  the  other  hand,  there  is  no  question 
as  to  the  adequacy  of  the  hypothesis  to  account  for  the 
whole  supply  of  solar  heat  ;  but  there  is  as  yet  no  direct 
evidence  whatever  that  the  sun  is  really  shrinking. 

The  basis  of  the  meteoric  theory  is  simply  this  :  If 
a  moving  body  be  stopped,  either  suddenly  or  gradually, 
a  quantity  of  heat  is  generated  which  may  be  expressed, 

in  calories,  by  the  formula  ,  in  which  m  is  the  mass 


of  the  body,  in  kilogrammes,  and  v  its  velocity,  in  me- 
tres per  second.  A  body  weighing  8,339  kilogrammes, 
and  moving  one  metre  per  second,  would,  if  stopped, 
develop  just  one  calory  of  heat  —  i.  e.,  enough  to  heat 
one  kilogramme  of  water  from  freezing-point  to  1° 
Cent.  If  it  were  moving  five  hundred  metres  per 
second  (about  the  speed  of  a  cannon-ball),  it  would  pro- 
duce two  hundred  and  fifty  thousand  times  as  much 
heat,  or  enough  to  raise  the  temperature  of  a  mass  of 
water  equal  to  itself  about  30°  Cent.  If  it  were  mov- 
ing, not  five  hundred  metres  per  second,  but  about  seven 
hundred  thousand  (approximately  the  velocity  with 
which  a  body  would  fall  into  the  sun  from  any  planet- 
ary distance),  the  heat  produced  would  be  1,400  x  1,400, 
or  nearly  two  million,  times  as  great  —  sufficient  to  bring 
a  mass  of  matter  many  thousand  times  greater  than  itself 
to  most  vivid  incandescence,  and  immensely  more  than 
could  be  produced  by  its  complete  combustion  under 
any  conceivable  circumstances.  Lord  Kelvin  (formerly 
Sir  William  Thomson)  has  calculated  the  amount  of 
heat  which  would  be  produced  by  each  of  the  planets 


THE  SUN'S  LIGHT  AND   HEAT.  313 

in  falling  into  the  sun  from  its  present  orbit.  The  re- 
sults are  as  follows,  the  heat  produced  being  expressed 
by  the  number  of  years  and  days  through  which  it 
would  maintain  the  sun's  present  expenditure  of  en- 
ergy: 

Years.  Days. 

Mercury 6  219 

Venus 83  326 

Earth 95     19 

Mars 12  259 

Jupiter 32,254 

Saturn 9,652 

Uranus 1,610 

Neptune 1,890 

Total 45,604 

That  is,  the  collapse  of  all  the  planets  upon  the  sun 
would  generate  sufficient  heat  to  maintain  its  supply  for 
nearly  forty-six  thousand  years.  A  quantity  of  matter 
equal  to  only  about  one  one-hundredth  of  the  mass  of 
the  earth,  falling  annually  upon  the  solar  surface,  would, 
therefore,  maintain  its  radiation  indefinitely.  Of  course, 
this  increase  of  the  sun  would  cause  an  acceleration  of 
the  motion  of  all  the  planets — a  shortening  of  their 
periods.  Since,  however,  the  mass  of  the  sun  is  three 
hundred  and  thirty  thousand  times  that  of  the  earth, 
the  yearly  addition  would  be  only  one  thirty-three-mill- 
ionth of  the  whole,  and  it  would  require  centuries  to 
make  the  effect  sensible.  The  only  question,  then,  is, 
whether  any  such  quantity  of  matter  can  be  supposed 
to  reach  the  sun.  "While  it  is  impossible  to  deny  this 
dogmatically,  it,  on  the  whole,  seems  improbable,  for 
astronomical  reasons.  In  the  first  place,  if  meteoric 
matter  is  so  abundant,  the  earth  ought  to  encounter 
much  more  of  it  than  she  does  ;  enough,  in  fact,  to 
raise  her  temperature  above  that  of  boiling  water. 


314  THE  SUN. 

Then,  again,  if  so  large  a  quantity  of  matter  annually 
falls  upon  the  solar  surface,  it  is  necessary  to  suppose 
a  vastly  greater  quantity  circulating  around  the  sun 
between  it  and  the  planet  Mercury.  The  process  by 
which  the  orbit  of  a  meteoric  body  is  so  changed  as 
to  make  it  enter  the  solar  atmosphere  is  a  very  slow 
one,  so  that  only  a  very  small  proportion  of  the  whole 
could  be  caught  in  any  given  year.  Now,  if  there  were 
near  the  sun  any  considerable  quantity  of  meteoric 
matter — anything  like  the  mass  of  the  earth,  for  in- 
stance— it  ought  to  produce  a  very  observable  effect 
upon  the  motions  of  the  planet  Mercury,  an  effect  not 
yet  detected.*  For  this  reason  astronomers  generally, 
while  conceding  that  a  portion,  and  possibly  a  consider- 
able fraction,  of  the  solar  heat  may  be  accounted  for  by 
this  hypothesis,  are  disposed  to  look  further  for  their 
explanation  of  the  principal  revenue  of  solar  energy. 
They  find  it  in  the  probable  slow  contraction  of  the 
sun's  diameter,  and  the  gradual  liquefaction  and  solidi- 
fication of  the  gaseous  mass.  The  same  total  amount 
of  heat  is  produced  when  a  body  moves  against  a  resist- 
ance which  brings  it  to  rest  gradually  as  if  it  had  fallen 
through  the  same  distance  freely  and  been  suddenly 
stopped.  If,  then,  the  sun  does  contract,  heat  is  neces- 
sarily produced  by  the  process,  and  that  in  enormous 
quantity,  since  the  attracting  force  at  the  solar  surface 
is  more  than  twenty-seven  times  as  great  as  gravity  at 
the  surface  of  the  earth,  and  the  contracting  mass  is  so 
immense. 

*  Leverrier  considered  that  he  had  detected  in  the  motions  of  Mercury 
an  irregularity  of  the  kind  indicated,  but  much  smaller.  It  was  such, 
according  to  his  calculations,  as  would  be  accounted  for  by  the  action  of 
one  or  several  planets  whose  aggregate  mass  should  be  much  less  than 
that  of  the  earth.  This  was  the  basis  on  which  he  founded  his  strong 
belief  in  the  existence  of  the  intra-Mercurial  planet  Vulcan. 


THE  SUN'S  LIGHT  AND  HEAT,  315 

In  this  process  of  contraction,  each  particle  at  the 
surface  moves  inward  by  an  amount  equal  to  the  whole 
diminution  of  the  solar  radius,  while  a  particle  below 
the  surface  moves  less,  and  under  a  diminished  gravi- 
tating force ;  but  every  particle  in  the  whole  mass  of 
the  sun,  excepting  only  that  at  the  exact  center  of  the 
globe,  contributes  something  to  the  evolution  of  heat. 
To  calculate  the  precise  amount  of  heat  developed,  it 
would  be  necessary  to  know  the  law  of  increase  of  the 
sun's  density  from  the  surface  to  the  center ;  but  Helm- 
holtz,  who  first  suggested  the  hypothesis,  in  1853,  has 
shown  that,  under  the  most  unfavorable  suppositions,  a 
contraction  *  in  the  sun's  diameter  of  about  three  hun- 
dred feet  a  year — a  mile  in  a  little  more  than  seventeen 
years — would  account  for  its  whole  annual  heat-emis- 
sion. This  contraction  is  so  slow  that  it  would  be  quite 
imperceptible  to  observation.  It  would  require  seven 
thousand  years  to  reduce  the  diameter  a  single  second  of 
arc,  and  nothing  less  would  be  certainly  detectable. 

Of  course,  if  the  contraction  is  more  rapid  than  this, 
the  mean  temperature  of  the  sun  must  be  actually  rising, 
notwithstanding  the  amount  of  heat  it  is  losing.  Obser- 
vation alone  can  determine  whether  this  is  so  or  not. 

If  the  sun  were  wholly  gaseous,  we  could  assert 
positively  that  it  must  be  growing  hotter ;  for  it  is  a 
most  curious  (and  at  first  sight  paradoxical)  fact,  first 
pointed  out  by  Lane  in  1870,  that  the  temperature  of 
a  gaseous  body  continually  rises  as  it  contracts  from 
loss  of  heat.  By  losing  heat  it  contracts,  but  the  heat 
generated  by  the  contraction  is  more  than  sufficient  to 
keep  the  temperature  from  falling.  A  gaseous  mass 
losing  heat  by  radiation  must,  therefore,  at  the  same 

•*  These  figures  have  been  altered  to  correspond  with  Langley's  yalue 
of  the  solar  constant. 


316  THE   SUN. 

time  grow  both  smaller  and  hotter,  until  the  density 
becomes  so  great  that  the  ordinary  laws  of  gaseous  ex- 
pansion reach  their  limit  and  condensation  into  the 
liquid  form  begins.  The  sun  seems  to  have  arrived 
at  this  point,  if  indeed  it  were  ever  wholly  gaseous, 
which  is  questionable.  At  any  rate,  so  far  as  we  can 
now  make  out,  the  exterior  portion — i.  e.,  the  photo- 
sphere— appears  to  be  a  shell  of  cloudy  matter,  precip- 
itated from  the  vapors  which  make  up  the  principal 
mass,  and  the  progressive  contraction,  if  it  is  indeed  a 
fact,  must  result  in  a  continual  thickening  of  this  shell 
and  the  increase  of  the  cloud-like  portion  of  the  solar 
mass. 

This  change  from  the  gaseous  to  the  liquid  form 
must  also  be  accompanied  by  the  liberation  of  an  enor- 
mous quantity  of  heat,  sufficient  to  materially  diminish 
the  amount  of  contraction  needed  to  maintain  the  solar 
radiation. 

Evidently  if  this  theory  of  the  source  of  the  solar 
heat  is  correct,  it  follows  that  in  time  it  must  come  to 
an  end ;  and  looking  backward  we  see  that  there  must 
also  have  been  a  beginning.  Time  was  when  there  was 
no  such  solar  heat  as  now,  and  the  time  must  come  when 
it  will  cease. 

We  do  not  know  enough  about  the  amount  of  solid 
and  liquid  matter  at  present  in  the  sun,  or  of  the  nature 
of  this  matter,  to  calculate  the  future  duration  of  the 
sun  with  great  exactness,  though  an  approximate  esti- 
mate can  be  made.  The  problem  is  a  little  complicated, 
even  on  the  simplest  hypothesis  of  purely  gaseous  con- 
traction, because  as  the  sun  shrinks  the  force  of  gravity 
increases,  and  the  amount  of  contraction  necessary  to 
generate  a  given  amount  of  heat  becomes  less  and  less  ; 
but  this  difficulty  is  easily  met  by  a  skillful  mathema- 


THE   SUN'S  LIGHT  AND  HEAT.  317 

tician.  According  to  Newcomb,  if  the  sun  maintains 
its  present  radiation  it  will  have  shrunk  to  half  its  pres- 
ent diameter  in  about  five  million  years  at  the  long- 
est. As  it  must,  when  reduced  to  this  size,  be  eight 
times  as  dense  as  now,  it  can  hardly  then  continue  to 
be  mainly  gaseous,  and  its  temperature  must  have  begun 
to  fall.  Newcomb's  conclusion,  therefore,  is  that  it  is 
hardly  likely  that  the  sun  can  continue  to  give  sufficient 
heat  to  support  life  on  the  earth  (such  life  as  we  now 
are  acquainted  with,  at  least)  for  ten  million  years  from 
the  present  time. 

It  is  possible  to  compute  the  past  of  the  solar  his- 
tory  upon  this  hypothesis  somewhat  more  definitely 
than  the  future.  The  present  rate  of  contraction  being 
known,  and  the  law  of  variation,  it  becomes  a  purely 
mathematical  problem  to  compute  the  dimensions  of 
the  sun  at  any  date  in  the  past,  supposing  its  heat-radi- 
ation to  have  remained  unchanged.  Indeed,  it  is  not 
even  necessary  to  know  anything  more  than  the  present 
amount  of  radiation,  and  the  mass  of  the  sun,  to  com- 
pute how  long  the  solar  fire  can  have  been  maintained, 
at  its  present  intensity,  by  the  process  of  condensation. 
No  conclusion  of  geometry  is  more  certain  than  that 
the  contraction  of  the  sun  from  a  diameter  even  many 
times  larger  than  that  of  Neptune's  orbit  to  its  present 
dimensions,  if  such  a  contraction  has  actually  taken 
place,  has  furnished  about  eighteen  million  times  as 
much  heat  as  the  sun  now  supplies  in  a  year ;  and  there- 
fore that  the  sun  can  not  have  been  emitting  heat  at 
the  present  rate  for  more  than  that  length  of  time,  if  its 
heat  has  really  been  generated  in  this  manner.  If  it 
could  be  shown  that  the  sun  has  been  shining  as  now  for 
a  longer  time  than  that,  the  theory  would  be  refuted ; 
but  if  the  hypothesis  be  true,  as  it  probably  is  in  the 


318  THE  SUN. 

main,  we  are  inexorably  shut  up  to  the  conclusion  that 
the  total  life  of  the  solar  system,  from  its  birth  to  its 
death,  is  included  in  some  such  space  of  time  as  thirty 
million  years.  No  reasonable  allowances  for  the  fall  of 
meteoric  matter,  based  on  what  we  are  now  able  to  ob- 
serve, or  for  the  development  of  heat  by  liquefaction, 
solidification,  and  chemical  combination  of  dissociated 
vapors,  could  raise  it  to  sixty  million. 

At  the  same  time,  it  is  obviously  impossible  to  assert 
that  there  has  been  no  catastrophe  in  the  past — no  col- 
lision with  some  wandering  star,  endued,  as  Croll  has 
supposed,  like  some  of  those  we  know  of  now  in  the 
heavens,  with  a  velocity  far  surpassing  that  to  be  ac- 
quired by  a  fall  even  from  infinity,  producing  a  shock 
which  might  in  a  few  hours,  or  moments  even,  restore 
the  wasted  energy  of  ages.  Neither  is  it  wholly  safe  to 
assume  that  there  may  not  be  ways,  of  which  we  yet 
have  no  conception,  by  which  the  energy  apparently 
lost  in  space  may  be  returned,  at  least  in  part,  and  so 
the  evil  day  of  the  sun's  extinction  may  be  long  post- 
poned. 

In  1882  Dr.  C.  W.  Siemens,  of  London,  proposed  a 
new  theory  of  the  solar  energy  much  in  this  line,  and 
the  scientific  eminence  of  its  author  secured  it  most  re- 
spectful consideration  and  discussion.  Although  it  was 
soon  abandoned  as  untenable,  on  account  of  want  of 
evidence  as  to  some  of  its  fundamental  assumptions, 
and  fatal  objections  to  it  from  astronomical  consider- 
ations, it  is  so  suggestive  and  instructive  that  we  have 
concluded  to  retain  the  statement  and  discussion  of  it 
that  was  given  in  a  supplementary  note  to  the  second 
edition  of  this  book  a  dozen  years  ago. 

"The  fundamental  conditions"  of  Dr,  Siemens's 
theory  are  the  following,  in  his  own  words  r^ 


THE  SUN'S  LIGHT  AND  HEAT.  319 

"  1.  That  aqueous  vapor  and  carbon  compounds  are  present 
in  stellar  and  interplanetary  space. 

"2.  That  these  gaseous  compounds  are  capable  of  being  disso- 
ciated (decomposed  into  their  elements)  by  radiant  solar  energy 
while  in  a  state  of  extreme  attenuation. 

"  3.  That  these  dissociated  vapors  are  capable  of  being  com- 
pressed into  the  solar  atmosphere  by  a  process  of  interchange 
with  an  equal  amount  of  reasaociated  vapors,  the  interchange 
being  effected  by  the  centrifugal  action  of  the  sun  itself." 

Granting  these  conditions,  he  argues  that  the  solar 
heat  is  produced  by  the  recombination  of  the  elemental 
gases  contained  in  a  current  which  is  continually  drawn 
in  upon  the  region  of  the  sun's  poles ;  this  current  be- 
ing produced  by  the  rotation  of  the  sun,  which  acts  like 
a  gigantic  fan-wheel,  throwing  off  the  adherent  gases 
at  its  equator,  and  thus  continually  removing  the  prod- 
ucts of  the  combustion,  and  redistributing  them  through 
space. 

Moreover — and  this  is  the  point  of  the  theory  upon 
which  he  puts  special  emphasis — he  teaches  that  these 
compound  gases  resulting  from  the  combustion  inter- 
cept the  solar  heat  not  received  by  the  planets  (heat 
which,  from  the  human  point  of  view,  would  otherwise 
be  wasted),  and  utilize  it  in  their  own  decomposition ; 
thus  the  solar  fire  is  made  to  prepare  its  own  fuel  from 
the  ashes  of  its  own  furnace,  and  an  explanation  is  found 
for  its  enduring  constancy. 

While,  for  reasons  soon  to  be  stated,  we  can  not  ac- 
cept this  theory,  it  may  be  said,  in  the  first  place,  that 
there  is  nothing  absurd  in  it.  It  is  not  to  be  put  into 
the  category  of  the  speculations  which  explain  gravita- 
tion and  the  planetary  motions  by  electric  vortices,  or 
any  similar  nonsense. 

IF  space  is  filled  with  composite  vapors,  and  IF  rays 
of  light  and  heat  can  decompose  them  again  into  their 


320  THE  SUN. 

elements,  then,  to  some  extent,  the  theory  not  only  may 
be  but  must  be  true.  A  hot  revolving  globe,  moving 
in  a  space  filled  with  such  vapors,  must  necessarily  pro- 
duce such  currents  as  Dr.  Siemens  indicates,  and  must 
maintain  a  continual  fire  upon  its  surface :  the  question 
would  only  be  how  great  and  how  hot  a  fire. 

Now,  as  to  Dr.  Siemens's  fundamental  hypotheses. 
Probably  no  one  will  care  to  deny  the  possible  existence 
of  some  gaseous  matter  in  space,  though  it  seems  much 
more  probable  that  what  matter  is  scattered  about  be- 
tween the  planets  is  mostly  in  the  form  of  little  solid 
grains,  such  as  we  see  from  time  to  time  in  shooting-stars. 

But  an  interplanetary  atmosphere  of  any  sensible 
density  is,  we  think,  irreconcilable  with  the  observed 
motions  of  planets,  and  especially  of  comets.  Dr.  Sie- 
mens suggests  -J^VF  of  the  density  of  the  earth's  atmos- 
phere as  a  possible  maximum,  without  indicating  what 
the  minimum  might  be.  But,  in  order  to  supply  the 
requisite  amount  of  material  to  keep  up  the  sun's  heat, 
the  density  of  the  interplanetary  atmosphere  would 
have  to  be  somewhere  from  ^-5-  to  T^Ainr  °^  *^e  den- 
sity  of  the  air  at  the  earth's  surface.* 

*  In  an  article,  contained  in  the  "  Nineteenth  Century "  for  April, 
1882,  Dr.  Siemens,  meeting  some  objections  that  had  been  made  to  his 
theory,  shows  that  a  current  of  mingled  gases,  containing  five  per  cent. 
of  nncombined  hydrogen  and  marsh-gas,  and  ninety-five  per  cent,  of  oxy- 
gen, nitrogen,  and  neutral  gas,  would  by  its  combustion  account  for  the 
whole  supply  of  solar  heat,  if  at  the  sun's  surface  the  density  were  the 
same  as  that  of  the  earth's  atmosphere,  and  the  velocity  a  hundred  feet 
per  second.  Taking  this  estimate,  and  assuming  that  the  inrushing  cur- 
rent continues  to  be  perceptible  at  a  distance  even  so  great  as  fifty  mill- 
ions of  miles,  and  that  its  filaments  move  toward  the  sun  in  converging 
lines,  we  find  that  the  interplanetary  atmosphere  must  there  have  a  den- 
sity of  about  TTOTHF  of  the  air  at  the  earth's  surface.  If  we  suppose  the 
indraught  to  vanish  at  a  less  distance  from  the  sun,  the  density  must  be 
greater. 


THE  SUN'S  LIGHT  AND  HEAT.  321 

Now  the  resistance  of  an  atmosphere,  of  even  the 
density  mentioned,  would  be  very  serious  upon  bodies 
moving  from  fifty  to  one  hundred  times  faster  than 
cannon-balls,  and  the  effect  could  not  fail  to  be  felt  in 
a  considerable  retardation  and  a  consequent  shorten- 
ing of  their  periods.  So  far  as  can  be  judged  from 
the  observed  behavior  of  Encke's  comet,  the  density 
of  the  interplanetary  medium  can  not  well  exceed 
^nr,Tnnr,i5i¥,innr,Tnnr  of  tne  earth's  atmosphere.  (See 
Harkness's  discussion  of  the  subject  in  the  Washington 
astronomical  observations  for  1870.) 

Siemens  indeed  remarks  that,  "assuming  that  the 
matter  filling  space  is  an  almost  perfect  fluid  not  lim- 
ited by  border  surfaces,  it  can  be  shown  that  the  retar- 
dation by  friction  through  such  an  attenuated  medium 
would  be  very  slight  indeed,  even  at  planetary  veloci- 
ties." But,  so  far  as  experiments  go,  the  rarefaction  of 
a  gas  does  not  tend  to  bring  it  more  nearly  to  the  con- 
dition of  a  perfect  fluid ;  on  the  other  hand,  it  seems 
to  approach  rather  to  the  condition  of  a  congeries  of 
minute  isolated  pellets  flying  through  space :  witness 
the  phenomena  of  the  radiometer  and  Crookes's  tubes. 

Another,  and  it  seems  to  us  quite  as  serious  an  ob- 
jection to  the  theory,  lies  in  the  fact  that  it  limits  the 
temperature  of  the  solar  surface  to  that  corresponding 
to  the  dissociation  point  of  the  gaseous  compounds 
there  formed  ;  and  this  dissociation  point  for  the  hydro- 
carbons and  the  vapor  of  water,  even  granting  a  con- 
siderable pressure  at  the  solar  surface,  is  not  above 
6,000°  to  8,000°  Fahr.,  which  is  much  below  the  tem- 
perature of  the  sun's  surface  indicated  by  all  the  most 
recent  determinations. 

Furthermore,  if  the  absorption  of  radiant  energy 
within  the  limits  of  the  solar  system  really  amounts  to 


322  »      THE  SUN. 

anything  sensible,  the  stars  ought  to  be  quite  invisible, 
or  at  any  rate  no  heat  should  reach  us  from  them. 

As  to  the  chemical  assumption  that  the  hydrocarbon 
compounds  when  greatly  rarefied  can  be  dissociated  by 
the  action  of  the  sun's  rays,  we  believe  that  no  evi- 
dence has  been  found  of  such  an  effect. 

And  yet  one  almost  regrets  that  the  theory  can  not 
be  accepted,  for  it  would  remove  some  very  serious  dif- 
ficulties that  now  embarrass  the  problem  of  the  evolu- 
tion of  our  planetary  system.  The  accepted  contraction 
theory  of  Helmholtz  certainly  appears  to  allow  too  little 
time  for  the  sun's  lifetime  of  radiant  activity  to  be  con- 
sistent with  a  reasonable  explanation  of  the  process  by 
which  the  present  state  of  things  has  come  about. 

While  we  have  mentioned  only  three  theories  of  the 
solar  heat,  the  reader  will  understand  that  a  multitude 
have  been  proposed  and  rejected,  some  as  absurd  and 
others  as  inadequate.  To  the  former  class  belong  the 
speculations  of  those  who  liken  the  sun  to  the  armature 
of  a  dynamo  or  the  whirling  plate  of  an  "  influence  ma- 
chine," forgetting  that  in  both  these  cases  the  energy 
radiated  as  light  and  heat  must  be  derived  ultimately 
from  the  sun's  energy  of  rotation ;  and  a  simple  calcu- 
lation shows  that  this  energy  of  rotation  is  not  suffi- 
cient to  maintain  the  radiation  for  even  one  hundred 
and  fifty  years. 

Those  theories,  on  the  other  hand,  that  seek  to  ac- 
count for  the  solar  heat  as  the  simple  cooling  of  an 
incandescent  body,  like  a  red-hot  ball  of  metal,  or  by 
the  "  combustion "  of  solar  material,  in  the  chemical 
sense  of  the  word,  or  by  the  simple  condensation  of 
vapors  into  clouds  and  the  liberation  of  the  so-called 
latent  heat  of  vaporization — these  all,  like  the  meteoric 
theory,  are  utterly  inadequate. 


CHAPTER  IX. 

SUMMARY  OF  FACTS,  AND  DISCUSSION  OF  THE  CONSTITUTION 

OF  THE  SUN. 

Table  of  Numerical  Data. — Constitution  of  Sun's  Nucleus. — Peculiar 
Properties  of  Gases  under  High  Temperature  and  Pressure. — Char- 
acteristic  Differences  between  a  Liquid  and  a  Gas. — Constitution  of 
the  Photosphere  and  Higher  Kegions  of  the  Sun's  Atmosphere. — 
Professor  Hastings's  Theory. — Pending  Problems  of  Solar  Physics. 

IT  may  be  well  to  collect  into  a  brief  summary  the 
principal  facts  and  conclusions  of  the  preceding  pages, 
presenting  them  in  a  single  comprehensive  view.  We 
give  first,  therefore,  a  table  of  the  statistics  of  the  sun 
— the  facts  which  can  be  stated  in  numbers : 

Solar  parallax  (equatorial  horizontal),  8-80"  ±  0'02". 

Mean  distance  of  the  sun  from  the  earth,  92,885,000  miles; 
149,480,000  kilometres. 

Variation  of  the  distanre  of  the  sun  from  the  earth  between  Janu- 
ary and  June,  3,100.000  miles;  4,950,000  kilometres. 

Linear  value  of  \"  on  the  sun's  surface,  450*3  miles;  724*7  kilo- 
metres. 

Mean  angular  semidiameter  of  the  sun,  16'  02-0"  ±  TO". 

Sun's  linear  diameter,  866,400  miles ;  1,394,300  kilometres.  (This 
may,  perhaps,  be  variable  to  the  extent  of  several  hundred 
miles.) 

Ratio  of  the  sun's  diameter  to  the  earth's,  109'5. 

Surface  of  the  sun  compared  with  the  earth,  11,940. 

Volume,  or  cubic  contents,  of  the  sun  compared  with  the  earth, 
1,305,000. 

Mass,  or  quantity  of  matter,  of  the  sun  compared  with  the  earth, 
331,000  ±  1,000. 

Mean  density  of  the  sun  compared  with  the  earth,  0-253. 

Mean  density  of  the  sun  compared  with  water,  1*406. 


324  THE  SUN. 

Force  of  gravity  on  the  sun's  surface  compared  with  that  on  the 
earth,  27'6. 

Distance  a  body  would  fall  in  one  second,  444'4  feet;  135-5  metres. 

Inclination  of  the  sun's  axis  to  the  ecliptic,  7°  15'.  ^| 

Longitude  of  its  ascending  node,  74°.  I  . 

Date  when  the  sun  is  at  the  node,  June  4-5.        f  ( 

Mean  time  of  the  sun's  rotation,  25-38  days. 

Time  of  rotation  of  the  sun's  equator,  25  days. 

Time  of  rotation  at  latitude  20°,  25-75  days. 

Time  of  rotation  at  latitude  30°,  26-5  days. 

Time  of  rotation  at  latitude  45°,  27'5  days. 

(These  last  four  numbers  are  somewhat  doubtful,  the  formulae 

of  various  authorities  giving  results  differing  by  several  hours  in 

some  cases.) 

Linear  velocity  of  the  sun's  rotation  at  his  equator,  1-261  miles 
per  second ;  2-028  kilometres  per  second. 

Total  quantity  of  sunlight,  1,575,000,000,000,000,000,000,000,000 
candles. 

Intensity  of  the  sunlight  at  the  surface  of  the  sun,  190,000  times 
that  of  a  candle-flame ;  5,300  times  that  of  metal  in  a  Bessemer 
converter;  146  times  that  of  a  calcium-light ;  3*4  times  that 
of  an  electric  arc. 

Brightness  of  a  point  on  the  sun's  limb  compared  with  that  of  a 
point  near  the  center  of  the  disk,  25  per  cent. 

Heat  received  per  minute  from  the  sun  upon  a  square  metre,  per- 
pendicularly exposed  to  the  solar  radiation,  at  the  upper  sur- 
face of  the  earth's  atmosphere  (the  solar  constant),  30  calories. 

Heat-radiation  at  the  surface  of  the  sun,  per  square  metre  per 
minute,  1,340,000  calories. 

Thickness  of  a  shell  of  ice  which  would  be  melted  from  the  sur- 
face of  the  sun  per  minute,  58-2  feet;  or  17*7  metres. 

Mechanical  equivalent  of  the  solar  radiation  at  the  sun's  surface, 
continuously  acting,  131,000  horse  power  per  square  metre; 
or  12,000  (nearly)  per  square  foot. 

Effective  temperature  of  the  solar  surface,  about  10,000°  Cent., 
or  18,000°  Fahr.  (according  to  Kosetti) ;  about  8,000°  Cent., 
or  14,400°  Fahr.  (according  to  Wilson  and  Gray). 

Of  course,  it  hardly  need  be  repeated  here  that  the 
figures  relating  to  the  light  and  heat  of  the  sun  are 


SUMMARY   OF  FACTS,   ETC.  325 

much  less  reliable  than  those  which  refer  to  its  distance, 
dimensions,  mass,  and  attracting  power. 

Fig.  100  is  intended  to  present  to  the  eye,  more 
clearly  than  any  mere  description,  the  constitution  of 
the  sun,  and  the  relation  of  the  different  concentric 
shells  or  envelopes  as  conceived  by  the  writer. 

The  picture  is  an  ideal  section  through  the  center. 
The  black  disk  represents  the  inner  nucleus,  which  is 
not  accessible  to  observation,  its  nature  and  constitution 
being  a  mere  matter  of  inference.  The  white  ring  sur- 
rounding it  is  the  photosphere,  or  shell  of  incandescent 
cloud  which  forms  the  visible  surface.  The  depth,  or 
thickness,  of  this  shell  is  quite  unknown ;  it  may  be 
many  times  thicker  than  represented,  or  possibly  some- 
what thinner.  Nor  is  it  certain  whether  it  is  separated 
from  the  inner  core  by  a  definite  surface,  or  whether, 
on  the  other  hand,  there  is  no  distinct  boundary  between 
them. 

The  outer  surface  of  the  photosphere,  however,  is 
certainly  pretty  sharply  defined,  though  very  irregular, 
rising  at  points  into  faculse,  and  depressed  at  others  in 
spots,  as  shown  in  the  figure. 

Immediately  above  this  lies  the  so-called  "  reversing 
stratum,"  in  which  the  Fraunhofer  lines  originate.  It 
is  to  be  noted,  however,  that  the  gases  which  compose 
this  stratum  do  not  merely  overlie  the  photosphere,  but 
they  also  fill  the  interspaces  between  the  photospheric 
clouds,  forming  the  atmosphere  in  which  they  float, 
and  an  attempt  has  been  made  to  indicate  this  fact  in 
the  diagram.  (See  page  326.) 

Above  the  "  reversing  stratum "  lies  the  scarlet 
chromosphere,  with  prominences  of  various  forms  and 
dimensions  rising  high  above  the  solar  surface  ;  and 
over,  and  embracing  all,  is  the  coronal  atmosphere  and 


326 


THE  SUN. 


the  mysterious  radiance  of  clouds,  rifts,  and  streamers, 
fading  gradually  into  the  outer  darkness. 

At  the  center  of  the  sun  the  earth  is  represented  in 
its  true  relative  dimensions — -    of  the  three  inches 


FIG.  100. 


which  is  taken  as  the  scale  of  the  sun's  diameter.  This 
scale  reduces  our  globe  to  a  little  dot  only  ^  of  an  inch 
across.  Around  it,  at  its  proper  distance,  is  drawn  the 
orbit  of  the  moon,  still  fur  within  the  photosphere,  the 
moon  herself  being  fairly  represented  by  any  one  of 


SUMMARY   OF  FACTS,  ETC.  327 

the  minute  points  which  make  up  the  dotted  line  that 
indicates  her  path. 

The  central  nucleus  is  made  black  in  the  picture, 
simply  for  convenience,  and  not  with  any  purpose  to 
indicate  that  the  matter  which  composes  it  is  cooler  or 
even  less  brilliantly  luminous  than  the  photosphere.  It 
is  quite  probable,  indeed,  that  this  central  core  (which 
contains  certainly  more  than  nine  tenths  of  the  whole 
mass  of  the  sun)  is  purely  gaseous,  and  it  is  of  course 
true  that,  at  a  given  temperature  and  pressure,  a  gaseous 
mass  has  a  lower  radiating  power,  and  is  less  luminous, 
than  a  mass  of  clouds,  such  as  those  which  constitute 
the  photosphere.  But,  on  the  other  hand,  both  com- 
pression and  increase  of  temperature  rapidly  raise  the 
radiating  power  of  a  gas  ;  and  it  is  highly  probable  that, 
at  no  very  considerable  depth,  the  growing  pressure  and 
heat  may  more  than  equalize  matters,  and  render  the 
central  nucleus  as  intensely  bright  as  the  surface  of  the 
sun  itself. 

At  the  upper  surface  of  the  photosphere,  however, 
and  all  through  it,  indeed,  the  uncondensed  gases  are 
dark  as  compared  with  the  droplets  and  crystals  which 
make  up  the  photospheric  clouds.  Here  the  pressure 
and  temperature  are  lowered,  so  that  the  vapors  give 
out  no  longer  a  continuous  but  a  bright-line  spectrum, 
whenever  we  get  a  chance  to  see  them,  against  a  non- 
luminous  background  ;  and,  when  the  intenser  light  from 
the  liquid  and  solid  particles  of  the  photosphere  shines 
through  these  vapors,  they  rob  it  of  the  corresponding 
rays,  and  produce  for  us  the  familiar  dark-lined  spec- 
trum of  ordinary  sunlight. 

It  is,  perhaps,  hardly  necessary  to  state  again  the 
reasons  for  believing  the  great  body  of  the  sun  to  be 
gaseous ;  the  argument  depends  upon  the  enormous 


328  THE  SUN. 

heat  at  the  surface,  which  keeps  the  solar  atmosphere 
charged  with  the  vapors  of  our  familiar  metals,  and  the 
fact  that  the  mean  density  of  the  sun  is  so  low  (only 
one  and  one  fourth  times  that  of  water),  that  it  is  quite 
impossible  that  any  of  the  substances  which  we  have 
reason  to  believe  to  exist  in  the  sun  could  have  the 
solid,  or  even  the  liquid,  form  through  any  considerable 
portion  of  its  mass.  That  is  to  say,  if  any  large  propor- 
tion of  the  whole  were  composed  of  solid  or  liquid  iron, 
titanium,  magnesium,  etc.,  the  density  would  be  far 
greater  than  it  really  is ;  and,  since  the  temperature,  at 
the  surface  even,  where  there  is  free  radiation  and  ex- 
posure to  the  cold  of  space,  is  so  high  as  to  keep  these 
bodies  in  the  state  of  vapor,  it  is  not  likely  that,  at 
greater  depths,  it  is  low  enough  to  permit  their  lique- 
faction or  solidification. 

And  yet  the  theory  that  th*ey  are  in  a  gaseous  state 
is  not  free  from  difficulties.  A  few  years  ago  it  would 
have  been  urged  with  great  plausibility  that,  under  the 
enormous  pressure  due  to  the  weight  of  the  superin- 
cumbent mass  acted  upon  by  the  solar  gravity — nearly 
twenty-eight  times  that  of  the  earth,  it  is  to  be  remem- 
bered— any  gas  whatever  must  be  liquefied  at  no  very 
great  depth  below  the  surface. 

Even  on  the  earth,  for  example,  the  density  of  the 
air  decreases  one  half  for  every  three  and  a  half  miles 
of  elevation,  and  it  ought  to  increase  in  a  similar  pro- 
portion for  every  three  and  a  half  miles  of  descent  be- 
low the  sea-level,  if  we  drop  for  a  moment  consideration!? 
relative  to  temperature.  Since  water  is  about  seven 
hundred  and  seventy  times  as  heavy  as  the  air  at  the 
earth's  surface,  it  follows,  therefore,  that  at  the  bottom 
of  a  shaft  thirty-five  miles  deep  the  air  would  be  more 
dense  than  water,  i*  of  the  same  temperature  as  at 


SUMMARY   OF  FACTS,   ETC.  329 

the  surface;  and,  before  a  depth  of  fifty  miles  were 
reached,  it  would  become  denser  than  gold,  unless  it 
had  first  liquefied,  and  so  become  less  compressible. 
If  we  take  account  of  the  slight  decrease  of  the  force  of 
gravity  as  we  go  below  the  earth's  surface,  and  assume 
that  the  temperature  increases,  even  at  the  rate  of  100° 
Fahr.  for  each  mile  of  descent,  the  results  will  be  modi- 
fied, but  not  materially  changed  in  character.  It  would 
merely  be  necessary  to  go  some  ten  miles  deeper  to 
reach  the  same  result. 

Now,  at  the  sun,  where  the  action  of  gravity  is  so 
much  more  intense,  it  is  evident  that,  unless  the  tem- 
perature rises  very  rapidly  below  the  surface,  or  unless 
liquefaction  supervenes,  the  density  of  gases  must  in- 
crease so  fast  that  the  mean  density  of  the  mass — if  the 
sun  be  really  gaseous — must  be  vastly  greater  than  that 
of  any  known  metal. 

But  liquefaction,  as  we  now  know,  can  not  take 
place  under  the  circumstances.  The  researches  of  An- 
drews and  his  successors  have  shown  that  to  liquefy 
a  gas  two  things  must  go  together — increase  of  pressure 
and  diminution  of  temperature.  For  each  gas  there  is 
a  so-called  "  critical  temperature,"  and,  so  long  as  the 
temperature  does  not  fall  below  this  point,  no  pressure 
whatever  can  reduce  the  gas  to  the  liquid  form.  When 
the  temperature  has  fallen  below  it,  then  pressure  alone 
will  produce  the  desired  effect,  and,  if  the  temperature  is 
very  low,  only  a  slight  degree  of  pressure  will  be  neededc 
Now  on,  or  in,  the  sun  the  temperature  can  not  be 
supposed  to  be  below  the  "  critical  points "  of  several 
/>f  the  gases  found  there,  and  hence,  as  has  been  said, 
their  liquefaction  is  out  of  the  question.  Those,  there- 
fore, who  are  unwilling  to  admit  a  sufficient  increase  of 
temperature  with  increasing  depth  below  the  solar  sur- 


330  THE  SUN. 

face,  have  been  disposed  to  hold  that  the  central  por 
tions  of  the  sun  are  not  composed,  to  any  great  extent, 
of  the  same  elements  which  the  spectroscope  reveals  to 
us  in  the  solar  atmosphere,  but  of  some  different  un- 
known solid  or  liquid  substance,  of  great  rigidity  and 
low  density.  With  this  view,  generally,  also  goes  the 
belief  that  the  evolution  of  solar  heat  is  essentially  a 
surface-action,  produced,  by  some  unexplained  process, 
only  where  the  exterior  of  the  solar  orb  encounters  open 
space,  and  not  of  necessity  implying  any  great  heat  in 
the  inner  depths.  The  older  observers,  especially  the 
Herschels,  for  the  most  part  held  theories  essentially 
like  that  sketched  above.  The  elder  Herschel,  it  will 
be  remembered,  even  contended  pretty  vigorously  that 
the  central  globe  of  the  sun  is  a  habitable  world,  shel- 
tered from  the  blazing  photosphere  by  a  layer  of  cool, 
non-luminous  clouds.  And  in  more  recent  times  Kirch- 
hoff  and  Zollner  have  maintained  that  the  luminous 
surface  is  either  liquid  or  solid. 

While  it  is,  perhaps,  not  possible  to  demonstrate  at 
present  the  falsity  of  this  theory,  by  proving  that  the 
solar  nucleus  is  neither  solid  nor  liquid,  and  showing 
that  the  solar  heat  is  not  confined  to  the  surface,  but 
permeates  the  whole  mass  with  continually  increasing 
intensity  near  the  center  of  the  globe,  it  is  yet  evident 
enough  that  it  meets  the  exigencies  of  the  case  only  by 
calling  in  unknown  and  imaginary  substances  and  op- 
erations. On  the  other  hand,  the  gaseous  theory,  which 
is  now  generally  adopted,  involves  no  new  kinds  of  mat- 
ter or  unknown  forces,  but  conceives  of  solar  phenom- 
ena as  entirely  the  same  in  kind  as  those  we  are  famil- 
iar with  in  our  laboratories,  though  immensely  different 
in  degree  and  intensity. 

If  we  only  grant  that  the  temperature  rises  rapidly 


SUMMARY   OF  FACTS,  ETC.  331 

enough  from  the  surface  downward  through  the  solar 
globe,  the  whole  difficulty  as  to  the  density  of  such  a 
gaseous  sphere  vanishes.  It  is  true  that,  on  this  view, 
the  central  temperature  must  be  tremendous,  even  in 
comparison  with  that  of  the  photosphere.  But  why 
not  ?  Can  any  reason  be  assigned  to  the  contrary  ?  If 
we  could  suppose  the  sun  wholly  made  of  hydrogen, 
and  that  the  ordinary  relations  deduced  by  our  labora- 
tory experiments  hold  between  the  pressure  and  tem- 
perature through  all  possible  ranges  of  both,  it  would 
then  be  a  comparatively  simple  matter  to  compute  the 
least  central  temperature  which  would  give  the  solar 
globe  its  present  density.  If,  however,  we  remember 
that  other  materials,  and  in  unknown  proportions,  enter 
into  the  problem,  and  that  in  all  probability  our  labora- 
tory-work gives  only  approximate  formulae,  it  is  clear 
that  such  a  computation  would  be  useless.  We  must 
content  ourselves  for  the  present  with  vague  expres- 
sions, and  say  roughly  that  the  intensity  of  the  sun's 
internal  heat  must  as  much  exceed  that  of  the  photo- 
sphere as  this  surpasses  the  mere  animal  warmth  of  a 
living  body. 

But  while,  on  the  whole,  it  thus  seems  probable  that 
the  sun's  core  is  gaseous,  nothing  could  be  remoter  from 
the  truth  than  to  imagine  that  a  mass  of  gas,  under 
such  conditions  of  temperature  and  pressure,  would  re- 
semble our  air  in  its  obvious  characteristics.  It  would 
be  denser  than  water  ;  and  since,  as  Maxwell  and  others 
have  shown,  the  viscosity  of  a  gas  increases  fast  with 
rising  temperature,  it  is  probable  that  it  would  resist 
notion  something  like  a  mass  of  pitch  or  putty. 

One  might,  then,  naturally  enough  ask,  why  a  sub- 
stance so  widely  different  from  gases  as  we  know  them 
by  experience,  and  so  much  resembling  what  we  are 


332  THE   SUN. 

accustomed  to  call  the  semifluids,  should  not  be  classed 
with  them  rather  than  with  the  gases.  The  reply,  of 
course,  is,  that  although  the  substance  thus  bears  a 
superficial  likeness  to  the  semifluids,  its  essential  char- 
acteristics are  still  those  of  a  gas,  viz.,  continuous  expan- 
sion under  diminishing  pressure  without  the  formation 
of  a  free  surface  of  equilibrium  ;  continuous  expansion 
under  increasing  temperature  without  the  attainment  of 
a  boiling-point ;  and,  in  the  case  of  a  mixture  of  dif- 
ferent gases,  a  uniform  diffusion  of  each,  according  to 
Dalton's  law,  without  regard  to  specific  gravity. 

Perhaps  a  little  fuller  explanation  may  be  allowed  on 
these  points,  which  are  often  misunderstood.  Suppose 
a  mass  of  liquid  to  be  contained  in  a  close  vessel,  which 
it  just  fills,  and  compressed  by  some  enormous  force ; 
now  let  the  vessel  grow  gradually  larger,  thus  relieving 
the  pressure.  The  liquid  will  expand,  at  first  keeping 
the  vessel  full ;  but  at  last,  even  if  heat  be  supplied  to 
prevent  the  temperature  from  falling,  a  time  will  come 
when  the  liquid  will  no  longer  fill  the  vessel,  but  an 
empty  space  will  be  left  above  a  well-defined  "  free  sur- 
face of  equilibrium" — a  space  empty,  that  is,  of  the 
liquid,  but  of  course  occupied  by  its  vapor.  Now,  if 
we  take  a  similar  vessel  filled  with  a  compressed  gas, 
the  density  of  which  may,  on  account  of  the  pressure, 
at  first  even  exceed  that  of  the  liquid  in  the  case  just 
cited,  and  allow  the  vessel  to  expand  in  the  manner  de- 
scribed, at  the  same  time  supplying  heat  enough  to  keep 
the  temperature  from  falling,  the  gas  will  never  cease 
to  fill  the  whole  vessel,  nor  will  it  ever  form  a  free  sur- 
face like  the  liquid,  however  far  the  enlargement  of  the 
vessel  may  be  carried. 

Again,  if  we  take  a  cylinder  with  a  weighted  piston, 
fitting  it  and  moving  freely  in  it?  and,  after  filling  the 


SUMMARY  OF  FACTS,  ETC.  333 

below  the  piston  with  a  liquid,  apply  heat  to  it, 
we  shall  find  that  at  first  the  temperature  will  rise  regu- 
larly, and  the  liquid,  expanding  slightly  as  it  warms, 
will  push  the  piston  before  it.  But,  when  a  certain 
temperature,  depending  upon  the  nature  of  the  liquid 
and  the  pressure  exerted  by  the  piston,  has  been  reached, 
the  liquid  will  cease  to  grow  hotter  by  the  further  ap- 
plication of  heat,  and  will  begin  to  boil ;  and  the  liber- 
ated vapor  will  raise  the  piston  and  occupy  the  otherwise 
vacant  space  above  the  surface  of  the  liquid.  If,  how- 
ever, the  space  originally  below  the  piston  were  occupied 
by  a  gas,  however  dense,  no  such  thing  would  happen. 
The  gas,  on  the  application  of  heat,  would  rise  in  tem- 
perature, and  expand  regularly  without  discontinuity  or 
limit. 

Finally,  as  to  the  third  criterion  which  marks  the 
difference  between  liquids  and  gases.  In  a  mixture  of 
liquids  of  different  specific  gravities,  the  different  mate- 
rials separate  and  arrange  themselves  in  strata,  according 
to  their  weights,  unless  they  have  some  chemical  action 
on  each  other — for  example,  quicksilver,  water,  and  oil. 
But  a  mixture  of  several  gases,  differing  however  widely 
in  specific  gravity — for  example,  hydrogen,  oxygen,  and 
carbon  dioxide — behaves  in  no  such  way  :  under  all  con- 
ditions of  temperature  and  pressure  each  gas  distributes 
itself  through  the  whole  space,  precisely  as  if  the  others 
were  not  present,  only  more  slowly  than  if  it  were  alone. 

Although  it  may  not  be  possible,  in  the  present  state 
of  science,  to  demonstrate  that  the  principal  portion  of 
the  solar  mass  is  gaseous,  this  much  can  at  least  be  said — 
that  a  globe  of  incandescent  gas,  under  conditions  such 
as  have  been  intimated,  would  necessarily  present  just 
uuch  phenomena  as  the  sun  exhibits. 

On  the  outer  surface,  exposed  to  the  cold  of  space, 


334  THE 

the  rapid  radiation  would  certainly  produce  the  con- 
densation and  precipitation  into  luminous  clouds  of 
such  vapors  as  had  a  boiling-point  higher  than  that  of 
the  cooling  surface.  These  clouds  would  float  in  an 
atmosphere  saturated  with  the  vapors  from  which 
they  were  formed,  and  also  containing  such  other  va- 
pors as  were  not  condensed,  and  thus  the  peculiarities 
of  the  solar  spectrum  would  result.  On  the  other 
hand,  the  permanent  gases,  like  hydrogen — those  not 
subject  to  condensation  into  the  liquid  form  under 
the  solar  conditions — would  rise  to  higher  elevations 
than  the  others,  and  form  above  the  photosphere  just 
such  a  chromosphere  as  we  observe.  Whether,  from 
the  mere  assumption  of  such  a  constitution  for  the  sun, 
one  could  work  out,  a  priori,  the  phenomena  of  sun- 
spots  and  prominences,  is  indeed  doubtful ;  but  thus 
far  nothing  in  any  of  them  has  been  observed  which 
appears  to  be  inconsistent  with  this  view  of  the  subject 
— nothing,  we  say,  unless  it  should  turn  out,  as  was 
once  maintained,  that  the  solar  surface  possesses,  so  to 
speak,  "geographical"  characteristics,  evinced  by  the 
disposition  to  break  out  into  sun-spots  at  certain  fixed 
points — as  if  at  those  points  there  were  volcanoes  or 
something  of  the  sort.  Of  course,  the  fact  that  the 
spots  are  distributed  mainly  in  two  belts  parallel  to  the 
solar  equator,  involves  no  difficulty,  for  it  is  easy  to 
conceive  how,  in  more  than  one  way,  the  sun's  rotation 
might  lead  to  such  a  result :  but  peculiarities  perma- 
nently attaching  to  individual  points  on  the  solar  sur- 
face necessarily  imply  rigid  connections,  such  as  are 
inconsistent  with  the  theory  of  a  gaseous  or  even  of  a 
fluid  nucleus.  But  while,  as  has  been  pointed  out  on 
p.  150,  there  is  a  marked  tendency  in  spots  to  recur  at 
or  near  the  same  points  during  several  solar  revolutions, 


SUMMARY   OF  FACTS,   ETC.  335 

there  is  no  evidence  which  establishes  the  existence  of 
fixed  spot-centers  ;  and  the  idea  is  to  be  regarded  mere- 
ly as  a  relic  of  the  old  Herschellian  theory  of  a  solid 
sun.  Still  it  is  difficult  to  test  the  notion  conclusively, 
even  by  means  of  such  extended  observations  as  those 
of  Carrington  or  Spoerer,  or  the  auroral  periods  of 
Veeder,  since  the  time  of  rotation  of  the  solid  nucleus, 
if  it  exists  at  all,  is  unknown,  and  this  makes  the  discus- 
sion difficult  and  unsatisfactory. 

With  reference  to  the  cpnstitution  of  the  photo- 
sphere there  is  a  general  agreement  among  astronomers. 
A  few,  perhaps,  still  hold,  as  has  been  mentioned,  to  the 
idea  that  the  visible  surface  is  a  liquid  sheet,  while  some 
believe  that  it  is  purely  gaseous ;  but  the  whole  appear- 
ance of  things,  the  details  of  the  granulation,  the  phe- 
nomena of  spots  and  faculse,  the  mobility  and  variability 
of  the  floccules,  all  better  accord  with  the  theory  adopted 
in  these  pages,  which  is  a  necessary  consequence  of  the 
hypothesis  that  the  sun  is  principally  gaseous.  It  seems 
almost  impossible  to  doubt  that  the  photosphere  is  a 
shell  of  clouds.  As  to  the  precise  constitution  of  this 
shell,  however,  the  form  and  magnitude  of  the  compo- 
nent cloudlets,  the  chemical  elements  involved,  and  the 
temperature  and  pressure,  there  is  room  for  a  good  deal, 
of  uncertainty  and  difference  of  opinion.  The  more 
common  view,  apparently — the  one,  certainly,  which  the 
writer  has  hitherto  held — is,  that  the  clouds  are  formed 
mainly  by  the  condensation  of  the  substances  which  are 
most  conspicuous  in  the  solar  spectrum,  such  as  iron 
and  the  other  metals.  As  to  the  form  of  the  clouds, 
also,  it  has  usually  been  assumed  that,  as  a  consequence 
of  the  ascending  currents  by  which  they  are  formed, 
they  are  columnar,  their  height  being  much  greater 
than  their  other  dimensions. 


336  THE  SUN. 

Professor  Hastings  has  proposed  a  somewhat  differ- 
ent theory  (already  referred  to  on  page  282),  which 
avoids  some  of  the  difficulties  of  the  received  doctrine, 
though  not  without  encountering  others  which  seem 
just  as  formidable. 

One  main  peculiarity  is  the  assumption  that  the 
photospheric  "  clouds  "  are  formed  by  the  precipitation 
of  either  carbon,  silicon,  or  boron  (the  three  members 
of  the  carbon  group),  to  the  exclusion  of  other  sub- 
stances which  are  less  refractory  (have  lower  boiling- 
points),  and  therefore  escape  precipitation.  Those 
bodies  which  have  boiling-points  higher  than  that  of 
this  photosphere-element,  as  it  may  be  called,  will,  there- 
fore, not  exist  to  any  extent  in  the  vaporous  atmosphere, 
having  suffered  precipitation  before  they  reach  the  visi- 
ble surface.  Those  only  will  show  their  lines  in  the 
spectrum  which  have  lower  boiling-points,  and  so  do 
not  suffer  precipitation  at  the  temperature  of  the  photo- 
sphere. He  gave  this  as  the  reason  why  the  lines  of 
silicon,  etc.,  do  not  appear  in  the  solar  spectrum,  a  re- 
mark which  has  now  lost  its  force,  since  the  later  work 
of  Rowland  and  others  shows  that  the  lines  of  carbon 
and  silicon  are  really  present.  The  carbon  lines  are 
several  hundred  in  number,  but  not  conspicuous  because 
they  fall  in  the  violet  and  ultra-violet.  The  lines  of  sili- 
con are  strong,  but  not  numerous.  It  will  at  once  be  seen 
that,  if  this  view  is  true,  the  temperature  of  the  photo- 
sphere is  that  of  the  boiling-point  (under  the  local  con- 
ditions of  pressure)  of  the  silicon  or  carbon,  or  what- 
ever it  is  which  forms  the  clouds.  As  an  objection  to 
the  view,  it  immediately  occurs  to  one  that,  if  the  car- 
bon, for  instance,  is  precipitated  at  and  below  some 
special  elevation,  yet  the  vapors  of  iron,  sodium,  and  all 
the  other  solar  metals  will  rise  above  it,  and,  in  their 


SUMMARY  OF  FACTS,  ETC.  337 

turn,  will  find  a  level  and  temperature  of  precipitation ; 
so  that  the  photospheric  clouds,  instead  of  being  com- 
posed of  any  single  substance,  would  contain  all  which 
can  find  a  level  and  temperature  of  precipitation  any- 
where in  the  solar  atmosphere.  As  to  the  form  of  the 
floccules,  it  would  seem  that  the  successive  precipitation, 
at  different  levels  and  temperatures  of  different  ele- 
ments in  an  ascending  current,  must  result  in  clouds  of 
great  vertical  extent — "  columnar,"  as  we  have  called 
them.  Professor  Hastings  rather  demurs  to  this,  how- 
ever, saying  that  in  his  observations  he  has  met  "  noth- 
ing which  would  indicate  a  columnar  form  of  the  gran- 
ules under  ordinary  circumstances." 

As  regards  the  explanation  of  the  absorption  layer 
which  darkens  the  edge  of  the  sun,  and  the  theory  of 
sun-spots  and  their  penumbra,  we  give  his  own  words : 

"The  precipitated  material  rapidly  cools  on  account  of  its 
great  radiating  power,  and  forms  a  fog  or  smoke  which  settles 
slowly  through  the  spaces  between  the  granules,  until  revolatilized 
below.  It  is  this  smoke  which  produces  the  general  absorption 
at  the  limb,  and  the  *  rice-grain '  structure  of  the  photosphere. 

"Where  any  disturbance  tends  to  increase  a  downward  con- 
vection current,  there  is  a  rush  of  vapors  at  the  outer  surface  of 
the  photosphere  toward  this  point.  These  horizontal  currents  or 
winds  carry  with  them  the  cooled  products  of  precipitation,  which, 
accumulating  above,  dissolve  slowly  below  in  sinking.  This  body 
of  smoke  forms  the  solar  spot. 

"The  upward  convection  currents  in  the  region  of  the  spots 
are  bent  horizontally  by  the  centripetal  winds.  Yielding  their 
heat  now,  by  the  relatively  slow  process  of  radiation,  the  loci  of 
precipitation  are  much  elongated,  thus  giving  the  region  imme- 
diately surrounding  a  spot  the  characteristic  radial  structure  of 
the  penumbra. 

"This  conception  of  the  nature  of  the  penumbra  implies  a 
ready  interpretation  of  a  remarkable  phenomenon,  amply  attested 
by  the  most  skillful  observers,  and,  as  far  as  my  knowledge  goes, 
23 


338  THE 

wholly  unexplained,  namely,  the  brightening  of  the  inner  edge  of 
the  penumbra  in  every  well-developed  spot. 

"  This  interpretation  is,  perhaps,  most  readily  imparted  by  a 
comparison  of  the  hot  convection  currents  in  the  two  cases. 
When  the  convection  current  is  rising  vertically,  the  medium  is 
cooled  by  expansion  until  the  precipitation  temperature  is  reached, 
when  all  the  coudensible  material  appears  suddenly,  save  as  it  is 
somewhat  retarded  by  the  heat  liberated  in  the  act.  Immediately 
afterward  the  particles  become  relatively  dark  by  radiation.  In 
the  horizontal  currents  a  very  different  condition  of  things  ob- 
tains. Here  the  medium  does  not  cool  dynamically,  by  expansion, 
but  only  by  radiation ;  hence  (since  the  radiation  of  the  solid  par- 
ticles is  enormously  greater  than  that  of  the  supporting  gas)  prac- 
tically by  that  of  the  particles  themselves.  Thus,  after  the  first 
particle  appears,  it  must  remain  at  its  brightest  incandescence 
until  all  the  material  of  which  it  is  composed  is  precipitated. 
From  this  we  see  that  such  an  horizontal  current  must  increase 
gradually  in  brilliancy  to  its  maximum,  and  then  suddenly  dimin- 
ish— an  exact  accordance  with  the  facts  as  observed." 

The  idea  that  the  stratum  which  produces  the  general 
absorption  at  the  limb  of  the  sun  is  a  veil  of  "  smoke  " 
— i.  e.,  of  the  same  minute  particles  which  constitute 
the  photosphere,  but  cooled  to  relative  darkness — has 
been  already  alluded  to  in  a  preceding  chapter.  So  far 
as  we  know,  it  is  novel  and  valuable,  clearing  up  a  good 
many  embarrassing  difficulties.  It  is  so  obvious,  on 
reflection,  that  something  of  the  sort  must  accompany 
the  photosphere,  that  it  is  surprising  that  the  idea  had 
not  been  thought  of  before.  Of  course,  the  particles 
formed  by  condensation  must,  many  of  them  at  least, 
be  carried  by  the  ascending  currents  high  above  the 
point  of  their  formation,  and  cooled  so  much  as  to  be- 
come relatively  dark  in  comparison  with  the  more  vivid 
incandescence  of  the  regions  below,  just  as  the  ascend- 
ing particles  of  carbon,  unconsumed  and  cooled,  consti- 
tute the  smoke  of  a  fire. 


SUMMARY   OF  FACTS,   ETC.  339 

As  regards  the  explanation  of  spot  phenomena,  we 
see  no  special  advantage  in  the  idea  proposed.  The 
received  theory  regards  the  general  brightening  at  the 
inner  edge  of  the  penumbra  as  produced  by  the  con- 
vergence of  the  luminous  filaments,  rendered  horizontal 
by  the  indraught.  The  quasi-bulbous  termination  of 
the  filaments  occurs  only  occasionally,  and  is  probably 
merely  an  illusion  due  to  "irradiation."  As  already 
stated  on  page  126,  with  a  large  telescope,  and  under 
the  finest  optical  conditions,  these  "  bulbs  "  assume  the 
appearance  of  "fish-hooks,"  so  to  speak,  of  extreme 
brilliance.  And  it  is  difficult,  though  perhaps  not  im- 
possible, to  reconcile  the  smoke  theory  of  sun-spot  dark- 
ening with  the  observations  of  the  writer  and  Duner 
upon  the  sun-spot  spectrum  (page  132),  which  seem  to 
show  that  the  absorbing  medium  is  mainly  gaseous.  It 
may  be  that  such  a  smoke  as  the  theory  supposes  would 
carry  with  it  a  sufficient  quantity  of  cooled  vapors  to 
explain  the  spindle-shaped,  close-packed  dark  lines  that 
are  observed,  while  the  bright  lines,  here  and  there  ob- 
served, can  be  accounted  for  as  due  to  overlying  gases. 

The  idea  that  carbon  may  be  the  main  constituent 
of  the  photosphere  is  by  no  means  new :  it  was  first 
seriously  advanced,  we  believe,  by  Johnstone  Stoney,  of 
Dublin,  as  early  as  1867,  mainly  on  physico-chemical 
grounds,  and  is  enthusiastically  advocated  by  Sir  Robert 
Ball  in  his  recent  "  Story  of  the  Sun."  It  is  quite  pos- 
sible that  the  objection  based  upon  the  lower  condensa- 
tion temperature  of  iron  and  other  metallic  vapors  may 
be  fairly  met  by  such  considerations  as  explain  the  pres- 
ence of  a  certain  amount  of  water- vapor  above  the  clouds 
in  our  own  atmosphere. 

As  regards  the  "  reversing  stratum  "  very  little  need 
be  added.  Mr.  Lockyer  indeed  denies  its  existence — 


340  THE 

that  is,  in  the  sense  that  there  is  a  thin  stratum,  close 
above  the  surface  of  the  photosphere,  in  which  most  of 
the  dark  lines  of  the  solar  spectrum  originate.  He 
maintains,  on  the  contrary,  in  accordance  with  his  Ci  dis- 
sociation theory,"  that  certain  of  the  lines,  due  to  sub- 
stances the  most  nearly  elementary,  and  having  their 
molecules  in  the  highest  stage  of  dissociation,  originate 
only  deep  down  in  the  solar  atmosphere  where  the  heat 
is  most  intense ;  others,  due  to  vapors  with  molecules 
somewhat  less  simple,  have  their  birth  a  little  higher ; 
and  others  yet,  due  to  molecules  the  most  complex,  are 
produced  only  in  the  most  elevated  regions  of  the  solar 
atmosphere;  each  elevation  thus  being  responsible  for 
its  own  special  family  of  spectrum  lines. 

If,  however,  we  reject  this  theory  as  "  not  proven," 
we  get  results  not  very  different. 

The  vapors  of  the  photosphere  and  chromosphere 
are  not  to  be  thought  of  as  entirely  separate  and  dis- 
tinct. All  the  gases  are  found  together  in  the  inter- 
stices between  the  cloud-granules  of  the  photosphere — 
the  unknown  substance  which  produces  the  green  line 
in  the  spectrum  of  the  corona,  the  hydrogen,  the  cal- 
cium, and  helium  which  characterize  the  chromo- 
sphere, and  the  metallic  vapors  which  give  the  revers- 
ing layer  its  peculiar  properties — these  all  exist  together 
in  the  lower  depths,  unless,  indeed,  it  may  possibly  be 
the  case  that  at  the  greater  elevations  some  compound 
bodies  are  formed  which  can  not  exist  in  the  fiercer 
fires  below.  So  far  as  we  can  distinguish  between 
these  different  portions,  we  may  define  the  photosphere 
as  the  shell  within  which  precipitation  is  taking  place ; 
the  reversing  layer,  as  that  lowest  region  of  the  solar 
atmosphere  which  contains  sensibly  all  the  gases  indi- 
cated by  the  spectroscope  ;  the  chromosphere,  as  the 


SUMMARY   OF  FACTS,   ETC.  341 

region  of  hydrogen,  calcium,  and  helium;  and  the 
corona,  as  that  upper  domain  of  the  solar  atmosphere 
which  becomes  observable  only  during  solar  eclipses. 
But  the  coronal  gas  itself  is  most  conspicuous  and 
abundant  right  in  the  photosphere  and  reversing  layer, 
and  the  same  is  true  of  the  hydrogen  of  the  promi- 
.nences. 

It  is  well,  also,  to  bear  in  mind  that,  if  any  sub- 
stances decomposable  by  heat  exist  upon  the  sun  at  all, 
we  must  expect  to  find  them  in  the  higher  and  cooler 
regions  of  the  solar  atmosphere.  In  and  near  the  pho- 
tosphere, or  ilnderneath  it,  matter  must  be  in  its  most 
elemental  state. 

As  to  the  mechanism  of  the  chromosphere  and 
prominences,  if  we  may  use  the  expression,  much  cer- 
tainly remains  to  be  learned.  In  many  cases,  indeed, 
perhaps  in  most,  the  forms  and  behavior  of  the  protu- 
berances are  satisfactorily  enough  accounted  for  by  sup- 
posing that  the  heated  hydrogen  and  its  associate  vapors 
is  simply  forced  up  into  cooler  regions  by  pressure  from 
below — a  pressure  which  must  result  from  the  down- 
ward movement  of  the  great  mass  of  precipitated  mat- 
ter which  forms  the  photosphere.  But  evidently  this 
is  not  the  whole  story.  We  must  have  recourse  to  ideas 
of  a  different  order  to  account  for  the  somewhat  rare, 
but  still  really  numerous  and  well-authenticated  in- 
stances when  the  summits  of  prominences  have  been 
seen  to  rise  in  a  few  minutes  to  elevations  of  two  or 
three  hundred  thousand  miles,  the  upward  motion  being 
almost  visible  to  the  eye  at  the  rate  of  a  hundred  miles 
a  second  or  more. 

Very  perplexing,  also,  is  the  indubitable  fact  that 
clouds  of  this  prominence-matter  sometimes  gather  and 
form  without  any  apparent  connection  with  the  chromo- 


34:2  THE  SUN- 

sphere  below,  apparently  just  as  clouds  form  in  our  own 
atmosphere,  by  the  condensation  of  vapor,  before  invis- 
ible. On  the  whole,  it  looks  very  much  as  if  we  must 
regard  the  prominences  as  differing  from  the  surround- 
ing medium  mainly,  if  not  wholly,  in  their  luminosity 
— as  simply  superheated  portions  of  an  immense  atmos- 
phere. 

But,  then,  we  immediately  encounter  the  difficulties 
so  ably  urged  by  Lane,  Lockyer,  and  others,  that  the 
existence  of  hydrogen  of  any  appreciable  density,  at  the 
elevation  of  even  a  hundred  thousand  miles,  implies  a 
density  and  pressure  at  the  surface  of  the  photosphere 
so  high  as  to  be  entirely  inconsistent  with  the  spectro- 
scopic  phenomena  there  manifested — unless,  indeed, 
under  solar  conditions,  the  action  of  gravity  upon  the 
gases  of  the  solar  atmosphere  is  modified  by  some  re- 
pulsive force.  That  such  a  force  is  at  least  conceivable, 
is  obvious  from  the  behavior  of  the  tails  of  comets ; 
and  many  features  in  the  corona  point  in  the  same  di- 
rection. Of  its  nature  and  origin  we  can  not,  however, 
assert  anything  as  yet. 

Even  more  difficult  than  the  problem  of  the  chromo- 
sphere is  that  of  the  corona.  While  it  is  something  to 
know  that  the  phenomenon  is  mainly  solar,  and  that, 
therefore,  it  must  rank  in  magnitude  and  importance 
with  the  most  magnificent  of  natural  objects,  we  have 
yet  to  find  a  satisfactory  explanation  of  many  of  its 
most  obvious  features.  It  is  certainly  very  complex — 
matter  meteoric  and  matter  truly  solar ;  orbital  motion, 
solar  attraction,  atmospheric  resistance,  and  actions 
thermal,  electrical,  and  magnetic,  are  probably  all  com- 
bined. 

At  present  it  would  seem  that  the  most  important 
and  fundamental  problems  of  solar  physics  which  are 


SUMMARY   OF  FACTS,   ETC.  343 

now  pressing  for  solution  are  these  :  first,  a  satisfactory 
explanation  of  the  peculiar  law  of  rotation  of  the  sun's 
surface ;  second,  an  explanation  of  the  periodicity  of 
the  spots,  and  their  distribution  ;  third,  a  determination 
of  the  variations  in  the  amount  of  the  solar  radiation 
at  different  times  and  different  points  upon  its  surface ; 
fourth,  a  satisfactory  explanation  of  the  relations  of  the 
gases  and  other  matters  above  the  photosphere  to  the 
sun  itself — the  problem  of  the  corona  and  the  promi- 
nences ;  and,  fifth,  the  discovery  of  some  reasonable 
hypothesis  as  to  the  sun's  loss  of  heat  by  radiation 
which  would  reconcile  our  estimates  of  its  age  and 
probable  future  endurance  with  the  demands  of  evolu- 
tionary theories  of  the  planetary  and  stellar  systems. 

One  might  name  many  others  of  hardly  less  interest, 
such  as  that  which  has  to  do  with  the  intimate  connec- 
tion between  terrestrial  magnetism  and  the  condition 
of  the  solar  surface ;  but,  on  the  whole,  the  five  named 
seem  to  be  those  the  solution  of  which  would  most  ad- 
vance our  science.  Not,  of  course,  that  we  are  to  sup- 
pose that  even  their  solution  would  bring  us  in  sight  of 
the  end  or  limit  of  knowledge.  Each  onward  step  only 
opens  before  us  a  new,  wider,  and  more  magnificent 
horizon,  with  infinity  still  beyond  it. 


SUPPLEMENTARY  NOTE. 


HELIUM,  ITS  IDENTIFICATION  AND  PROPERTIES. 

WHILE  this  work  was  under  revision  and  passing 
through  the  press  fresh  announcements  respecting  heli- 
um followed  each  other  so  rapidly  that  it  soon  became 
evident  that  it  would  be  useless  to  attempt  to  include 
them  all  in  the  text,  and  that  the  better  course  would  be 
to  add  a  supplementary  note  which  should  represent  as 
nearly  as  possible  the  state  of  our  knowledge  at  the 
date  of  final  publication. 

The  famous  D3  line  was  first  seen  in  1868,  when  the 
spectroscope  was  for  the  first  time  directed  upon  a  solar 
eclipse.  Most  of  the  observers  supposed  it  to  be  the  D 
line  of  sodium,  but  Janssen  noted  its  non-coincidence ; 
and  very  soon,  when  Lockyer  and  Frankland  took  up 
the  study  of  the  chromosphere  spectrum,  they  found 
that  the  line  could  not  be  ascribed  to  hydrogen  or  to 
any  then  known  terrestrial  element.  As  a  matter  of 
convenient  reference  Frankland  proposed  for  the  un- 
known substance  the  provisional  name  of  "  helium " 
(from  the  Greek  "  helios,"  the  sun),  and  this  ultimately, 
though  rather  slowly,  gained  universal  acceptance. 

Within  a  year  two  other  lines  (X  7,065  and  X  4,472) 
were  discovered  in  the  chromosphere  spectrum  by  Rayet 
and  Respighi  respectively,  which  like  D8  are  always 


SUPPLEMENTARY  NOTE.  345 

present  in  the  prominences,  but  have  no  corresponding 
dark  lines  in  the  ordinary  solar  spectrum.  It  was  of 
course  early  suggested,  but  without  proof,  that  these 
lines  also  were  due  to  helium.  Since  then  some  eight 
or  ten  other  lines  have  been  found,  frequent!/,  but  not 
always,  presenting  themselves  in  the  chromosphere 
spectrum,  and,  like  the  first  three,  also  without  dark 
analogues.  Moreover,  still  more  recently,  D3  and  its 
congeners  have  been  detected  in  stellar  spectra — dark 
in  the  spectra  of  the  "  Orion  stars,"  bright  in  the  spectra 
of  certain  variables  and  of  the  so-called  Wolf-Rayet 
stars;  and  loth  bright  and  dark  in  ft  Lyrae  and  the 
"  new  star  "  of  Auriga  which  appeared  in  1892. 

Naturally  there  has  been  much  earnest  searching 
after  the  hypothetical  element,  but  until  very  recently 
wholly  without  success ;  though  it  should  be  mentioned 
that  in  1881,  Palmieri,  the  director  of  the  earthquake 
observatory  upon  Vesuvius,  announced  that  he  had 
found  D8  in  the  spectrum  of  one  of  the  lava  minerals 
with  which  he  was  dealing.  But  he  did  not  follow  up 
the  announcement  with  any  evidence,  nor  has  it  ever 
received  any  confirmation,  and  from  what  we  now  know 
as  to  the  conditions  necessary  to  bring  out  the  helium 
spectrum,  there  is  every  reason  to  suppose  that  he  was 
mistaken. 

The  matter  remained  a  mystery  until  April,  1895, 
when  Dr.  Ramsay,  who  was  Lord  Rayleigh's  chemical 
collaborator  in  the  discovery  of  argon,  in  examining  the 
gas  liberated  by  heating  a  specimen  of  Norwegian 
cleveite,  found  in  its  spectrum  the  D3  line,  conspicuous 
and  indubitable.  The  mineral  was  obtained  from  Hille- 
brand,  one  of  our  American  chemists,  who  had  pre- 
viously studied  it,  and  ascertained  that  it  could  be  made 
to  give  off  a  gas  which  he  identified  with  nitrogen.  It 


346  SUPPLEMENTARY   NOTE. 

really  was  nitrogen  in  part,  but  Ramsay  suspected  that 
he  should  also  find  argon,  as  he  did — and  helium  be- 
sides, which  was  unexpected. 

Cleveite  is  a  species  of  uraninite  or  pitch  blende,  and 
it  soon  appeared  that  helium  could  be  obtained  from 
nearly  all  the  uranium  minerals,  and  from  many  others ; 
from  many,  mingled  with  argon ;  from  others,  nearly 
pure.  In  fact,  it  turns  out  to  be  very  widely  distributed, 
though  only  in  extremely  small  quantities,  and  generally 
"  occluded,"  or  else  in  combination — seldom,  if  ever,  free. 
It  has  been  detected  in  meteoric  iron,  in  the  waters  of 
certain  mineral  springs  in  the  Black  Forest  and  Pyre- 
nees, and  Kayser  even  reports  traces  of  it  in  the  atmos- 
phere at  Bonn. 

It  is  generally  obtained  by  heating  the  substance  that 
contains  it  in  a  close  vessel  connected  with  an  air-pump 
of  some  kind  by  which  the  liberated  gases  are  drawn  off 
and  collected.  They  are  then  laboriously  treated  to 
remove  as  far  as  possible  all  the  foreign  elements  (nitro- 
gen, etc.),  since  the  presence  of  as  much  as  five  or  ten 
per  cent,  of  any  other  gas  prevents  the  new  elements 
from  giving  any  spectroscopic  evidence  of  their  pres- 
ence ;  they  are  too  shy  and  modest  to  obtrude  them- 
selves. In  many  cases,  as  has  been  said,  argon  and 
helium  come  off  together,  and  certain  lines  in  their 
spectrum  are  nearly  coincident,  so  that  for  a  time  there 
was  supposed  to  be  some  close  bond  of  connection  be- 
tween them.  The  latest  observations,  however,  make 
it  certain  that  this  is  not  so :  as  Mr.  Lockyer  puts  it, 
"  argon  is  of  the  earth,  earthy,  but  helium  is  distinctly 
celestial." 

Its  spectrum  has  been  thoroughly  studied  by  Crookes, 
Lockyer,  and  Runge,  who  agree  as  to  all  its  leading 
characteristics. 


SUPPLEMENTARY  NOTE.  347 

Runge,  whose  work  is  most  complete  and  authorita- 
tive, finds  that  its  lines  have  a  remarkably  regular  ar- 
rangement, falling  into  two  distinct  "sets,"  each  set 
consisting  of  a  principal  series  and  two  subordinate 
ones,  the  lines  in  each  series  corresponding  very  accu- 
rately to  a  formula  quite  similar  to  that  discovered  by 
Balmer  as  governing  the  hydrogen  spectrum. 

In  the  whole  spectrum  he  finds  (by  photography 
mainly)  sixty-seven  lines,  twenty  of  which  only  are  in 
the  visible  part  of  the  spectrum.  Of  the  sixty-seven, 
twenty-nine  belong  to  the  first  "  set "  and  thirty-eight 
to  the  second.  Of  the  twenty  "  visual "  lines,  thirteen 
have  been  observed  in  the  spectrum  of  the  chromo- 
sphere ;  the  missing  lines  all  belong  to  the  second  subor- 
dinate series  of  the  first  "  set,"  and  are  so  faint  in  the 
artificial  spectrum  of  the  gas  that  their  failure  to  be 
found  in  the  chromosphere  needs  no  explanation. 

The  fact  that  the  lines  thus  divide  into  two  mathe- 
matically independent  "  sets  "  has  led  Runge  to  believe 
that  the  helium  obtained  from  the  minerals  is  really  a 
mixture  of  two  distinct  gases,  and  he  has  found  it  pos- 
sible to  partially  separate  the  two  by  a  process  of  diffu- 
sion. The  true  helium,  the  one  that  gives  D,  and  the 
other  lines  that  are  always  present  in  the  chromosphere 
spectrum,  he  considers  to  be  the  denser  of  the  two ;  the 
spectrum  of  the  other  contains  most  of  the  lines  that 
appear  only  occasionally  in  prominences.  With  this 
view  Mr.  Lockyer  is  entirely  in  agreement.  The  lighter 
component  has  as  yet  received  no  name.  Lockyer  calls 
it  simply  X. 

The  lines  of  the  series  to  which  D,  belongs  are  all 
double,  having  a  very  faint  companion  on  the  lower 
(i.  e.,  red-ward)  side,  extremely  close  to  the  principal 
line.  When  Range  announced  this  discovery  early  in 


348  SUPPLEMENTARY  NOTE. 

June  it  at  first  produced  something  like  consternation 
among  spectroscopists,  for  at  that  time  there  still  re- 
mained more  or  less  doubt  as  to  the  validity  of  Ram- 
say's identification,  and  the  solar  D,  had  never  been 
observed  to  have  such  a  companion.  Very  soon,  how- 
ever, Hale,  Huggins,  Reed,  and  other  observers  who 
had  sufficiently  powerful  instruments,  detected  the  lit- 
tle attendant  of  D8  in  the  spectrum  of  prominences,  so 
that  the  momentary  distrust  was  replaced  by  absolute 
confidence. 

As  to  the  physical  and  chemical  properties  of  the 
new  gas  our  knowledge  is  still  limited  and  our  conclu- 
sions embarrassed  by  the  uncertainty  whether  we  are 
dealing  with  a  single  element  or  a  mixture — whether 
Dr.  Ramsay  has  introduced  to  the  world  one  infant  or 
a  pair  of  twins. 

The  gas  liberated  from  cleveite,  and  purified  as  far 
as  possible,  shows  a  density  just  a  little  more  than  double 
that  of  hydrogen,  and  is  therefore  much  lighter  than 
any  other  known  gas  except  hydrogen  itself.  If  it  is  a 
mixture  the  lighter  gas  must  have  a  density  less  than 
two,  and  may  even  prove  to  be  lighter  than  hydrogen, 
while  the  true  D3  helium  may  have  a  density  anywhere 
between  two  and  four,  depending  on  the  proportions  of 
the  mixture  and  the  density  of  the  lighter  compound. 
It  would  be  very  fine,  we  may  remark  in  passing,  if 
the  lighter  component  could  have  been  identified  with 
"  coronium,"  but  this  seems  impossible  since  the  char- 
acteristic 1,474:  line  (X  5,316)  does  not  appear  at  all  in 
the  spectrum  of  terrestrial  "  helium  "  derived  from  any 
source. 

Ramsay's  acoustic  experiments  tend  to  show  that 
helium,  like  argon,  is  monatomic,  but  can  hardly  be 
considered  conclusive.  If  he  is  right,  the  atomic  weight 


SUPPLEMENTARY  NOTE.  349 

of  helium  must  be  not  far  from  four ;  but  thus  far  all 
attempts  to  make  it  enter  into  chemical  combination 
have  failed,  though  it  seems  rather  probable  that  in  the 
uraninite  minerals  it  is  held  by  stronger  bonds  than 
those  of  mere  occlusion. 

Olszewski  has  tried  his  best  to  liquefy  the  gas,  but 
thus  far  unsuccessfully ;  the  methods  that  have  con- 
quered every  other  gas,  hydrogen  itself  included,  have 
failed  with  helium — a  circumstance  very  remarkable, 
since  generally  a  denser  gas  liquefies  more  easily  than  a 
lighter  one,  and  hitherto  hydrogen  has  stood  pre-emi- 
nent in  its  refractoriness. 

Probably  the  question  has  suggested  itself  to  every 
reader  how  it  happens  that  helium,  so  conspicuous  in 
the  atmosphere  of  the  sun  and  many  stars,  should  be  so 
nearly  absent  from  our  own  atmosphere  and  so  scantily 
present  in  any  form  upon  the  earth.  The  answer  seems 
to  depend  upon  two  facts — the  chemical  inertness  of 
the  substance  and  its  low  density. 

According  to  Johnstone  Stoney's  deductions  from 
the  accepted  theory  of  gases,  no  free  gas  of  low  density 
can  remain  permanently  upon  a  heavenly  body  of  small 
mass  and  habitable  temperature,  but  the  molecules  will 
fly  off  into  space.  A  particle  leaving  the  earth  with  a 
velocity  of  about  seven  miles  a  second  would  never  re- 
turn to  it.  Now,  according  to  the  dynamic  theory  of 
gases,  the  molecules  of  our  atmosphere  are  flying  swiftly 
about  with  velocities  (at  ordinary  temperatures)  of  from 
1,500  to  10,000  feet  per  second ;  the  heavier  molecules, 
like  those  of  oxygen  and  nitrogen,  move  comparatively 
slowly,  but  if  any  free  hydrogen  or  helium  is  present 
its  molecules  take  up  velocities  several  times  more  swift, 
and  any  that  may  happen  to  be  near  the  upper  limits  of 
the  atmosphere  would  be  likely  to  be  thrown  off  into 


350  SUPPLEMENTARY  NOTE. 

space.  In  the  case  of  the  moon  even  the  oxygen  and 
the  nitrogen  would  go,  since  she  is  so  small  that  a  ve- 
locity not  much  exceeding  a  mile  a  second  would  carry 
them  off.  If  this  is  correct  it  is  easy  .to  see  why  we 
now  have  no  appreciable  quantity  of  free  hydrogen  or 
other  light  gas  in  our  atmosphere. 

But  while  we  have  no  atmospheric  hydrogen  to 
speak  of,  hydrogen  in  combination  is  extremely  abun- 
dant ;  one  eighth  part  by  weight  of  all  the  water  in  the 
sea  is  hydrogen ;  and  hydrogen  combines  freely  with 
many  other  elements  besides  oxygen,  so  that  we  contin- 
ually liberate  it  in  all  sorts  of  chemical  decompositions. 
Helium,  on  the  other  hand,  enters  into  combination 
most  sparingly,  is  therefore  scarce,  and  even  when  pres- 
ent is,  as  we  have  said  before,  not  easy  to  detect. 

November,  1895. 

The  most  recent  investigations  with  respect  to  the 
Cleveite  gas,  while  not  even  yet  entirely  decisive,  rather 
favor  the  conclusion  that  it  is  a  single  element,  and  not 
a  mixture  of  two  or  more  (1897). 

(Supplementary  to  page  89.)  Carbon  lines  have  lately 
been  detected  in  the  spectrum  of  the  chromosphere  on 
photographs  made  by  Professor  Reed,  and  visually  by 
Professor  Hale  of  the  Yerkes  Observatory.  Many  high 
authorities  consider  that  the  "  carbon  bands "  found  in 
the  solar  spectrum  are  really  due  to  cyanogen,  a  compound 
of  carbon  and  nitrogen,  thus  adding  the  latter  element  to 
the  solar  list.  In  1896  Runge  discovered  also  in  the  solar 
spectrum  a  triplet  of  oxygen  lines,  which  appear  not  to  be 
telluric,  and  make  the  presence  of  oxygen  in  the  solar 
atmosphere  highly  probable,  if  not  absolutely  certain. 
(January,  1898). 


SUPPLEMENTARY  NOTES. 


NOTE  A. 

ON   THE   ADJUSTMENT   OF   THE    SPECTROSCOPE. 
Supplementary  to  page  74. 

THE  following  directions  are  given  in  response  to 
the  requests  of  a  number  of  the  readers  of  this  book : 

A.  The  first  step  is  to  focus  the  view-telescope  ap- 
proximately.    Remove  it  from  the  spectroscope,  and, 
turning  it  toward  the  sky,  adjust  the  eye-piece  until  the 
wires  of  the  micrometer,  if  there  is  one,  are  perfectly 
distinct ;  then  pointing  to  some  distant  object,  move  the 
micrometer  itself  in  or  out  until  the  object  is  distinct  at 
the  same  time  with  the  wires.     If  there  is  no  microm- 
eter, it  is  necessary  only  to  set  the  eye-piece  to  distinct 
vision  of  the  remote  object.     It  is  best  to  make  a  per- 
manent mark  of  some  kind  on  the  tube  by  which  the 
telescope  can  be  again  focused  at  once  if  it  happens  to 
be  disturbed. 

B.  To  focus  the  collimator.     Replace  the  view-tele- 
scope on  the  spectroscope,  and  if  the  construction  of 
the  instrument  permits,  remove  the  prisms  or  grating 
and  set  the  telescope  so  as   to  look   directly  into  the 
collimator.      Bring   the   collimator-slit   exactly   to  the 
center  of  the  field  of  view  and  close  it  so  as  to  make 
it  very  narrow.     Now,  being  careful  not  to  disturb  the 

351 


352  SUPPLEMENTARY   NOTES. 

focus  of  the  telescope,  slide  in  or  out  the  tube  that  car- 
ries the  slit  until  the  slit  appears  in  the  telescope  clear 
and  sharp  as  a  fine,  colorless  line  of  light.  It  may  be 
necessary  to  illuminate  the  slit  by  putting  a  piece  of 
white  paper  before  it,  or  in  some  other  way. 

It  frequently  happens  that  the  instrument  will  not 
allow  the  telescope  to  be  pointed  into  the  collimator. 
In  that  case  we  can  proceed  as  follows  if  the  spectro- 
scope has  a  grating:  Turn  the  grating  until  the  re- 
flected image  of  the  slit  is  thrown  into  the  view-tele- 
scope, and  then  focus  the  collimator  as  before  directed. 
(In  order  to  get  the  image  of  the  slit  into  the  center  of 
the  field  of  view  it  may  be  necessary  to  touch  the 
screws  which  adjust  the  inclination  of  the  grating.)  If 
the  grating  is  truly  flat,  this  method  will  give  just  as 
good  results  as  the  one  first  mentioned,  and  the  gratings 
now  obtainable  are  generally  unexceptionable  in  this 
particular. 

With  a  prism  spectroscope  the  best  that  can  usually 
be  done  is  simply  to  focus  the  collimator  by  sliding  in 
or  out  the  slit-carrier  until  the  lines  of  the  spectrum 
are  sharp  without  touching  the  focus  of  the  view-tele- 
scope. If,  however,  the  prism  surfaces  are  not  per- 
fectly flat,  as  often  happens,  no  satisfactory  result  can 
be  obtained  in  this  way,  and  the  prisms  are  unfit  for 
prominence  or  sun-spot  observations,  although  they  may 
answer  perfectly  for  some  kinds  of  laboratory  work. 

C.  Make  the  slit  parallel  to  the  lines  of  the  grating 
or  the  edges  of  the  prisms  by  twisting  the  tube  that 
carries  it  until  the  spectrum-lines  are  perpendicular  to 
the  edges  of  the  spectrum ;  this  is  usually  done  at  the 
time  when  the  collimator  is  focused. 

D.  To  make  the  lines  of  the  grating  perpendicular 
to  the  plane  which  contains  the  optical  axes  of  the  view- 


ADJUSTMENT  OF  SPECTROSCOPE.  353 

telescope  and  collimator.  If  the  adjustment  happens 
to  be  perfect,  then,  on  turning  the  grating,  the  spectra 
will  travel  straight  across  the  field  of  view  and  without 
either  rising  or  falling. 

If  the  spectra  run  straight,  but  rise  on  one  side  of 
the  slit  image  and  fall  on  the  other  as  the  grating  is 
turned,  the  fault  is  corrected  by  raising  one  edge  of  the 
grating  (so  as  to  rotate  it  slightly  in  its  own  plane)  by 
means  of  the  adjusting  screws.  If  the  spectra  run  in 
a  curve,  it  shows  that  the  plane  of  the  grating  is  not 
parallel  to  the  axis  of  rotation  and  the  top  of  the  grat- 
ing must  be  inclined  a  little  one  way  or  the  other.  It 
is  likely  to  require  several  trials  to  get  both  adjustments 
perfect,  as  touching  one  is  apt  to  disturb  the  other.  In 
spectroscopes  with  prisms  the  maker  is  supposed  to  have 
attended  to  the  corresponding  adjustment.  If  he  has 
not  done  so,  it  is  a  rather  serious  job  to  make  it  cor- 
rectly by  slightly  tipping  the  prisms,  and  it  would  take 
too  much  time  to  describe  the  method. 

E.  To  adjust  accurately  the  focus  of  both  view-tele- 
scope and  collimator  for  a  special  ray  of  the  spectrum. 
(The  focus  differs  somewhat  for  the  different  colors.) 
Suppose  the  C-line  is  in  question  :  Turn  the  grating  so 
as  to  bring  into  the  field  of  view  of  the  view-telescope 
the  C-line  in  the  spectrum  of  the  second  order  on  the 
more  dispersive  side  of  the  slit-image.  (This  spectrum 
has  the  Hue  end  between  the  observer  and  the  collimator 
as  he  puts  his  eye  to  the  eye-piece.)  Probably  he  will 
find  that  the  line  is  not  quite  sharp;  if  so,  make  it 
sharp  by  focusing  the  collimator  without  touching  the 
telescope-focus  as  determined  on  a  distant  object. 
Next  turn  the  grating  so  as  to  bring  in  the  second 
order  C  in  the  less  dispersed  spectrum  on  the  other 
side  of  the  slit-image.  The  telescope  will  be  now  per- 

24 


354  SUPPLEMENTARY   NOTES. 

haps  perceptibly  out  of  focus  ;  if  so,  focus  the  telescope 
this  time,  leaving  the  collimator  undisturbed.  Then  go 
back  again  to  the  more  dispersive  side  and  see  if  the  C- 
line  is  still  in  focus  ;  if  not,  readjust  the  collimator- 
focus  again.  It  will  need  only  a  touch.  Of  course,  if 
found  necessary,  a  third  approximation  can  be  made  in 
the  same  way.  Always  focus  the  collimator  on  the 
more  dispersed  spectrum  and  the  telescope  on  the  less 
dispersed,  alternating  back  and  forth  until  the  adjust- 
ment becomes  perfect. 

F.  To  adjust  the  slit  to  the  focal  plane  of  the  equa- 
torial to  which  the  spectroscope  is  attached.  Take  out 
the  eye-piece  of  the  equatorial,  turn  the  spectroscope 
around  until  the  slit  lies  nearly  north  and  south,  and 
direct  the  instrument  to  the  sun,  pointing  it  so  that  the 
northern  or  southern  edge  of  the  sun's  image  lies  across 
the  slit,  nearly  in  the  middle.  Use  a  dark  glass  in 
looking  at  the  image,  otherwise  the  eye  will  be  dazzled 
and  lose  much  of  its  sensitiveness.  Move  the  whole 
spectroscope  nearer  to  or  farther  from  the  object-glass 
of  the  equatorial  until  the  edge  of  the  image  appears 
to  be  sharp  on  the  slit-plate ;  this  will  give  an  approxi- 
mate adjustment.  Now  look  into  the  spectroscope  and 
bring  the  C-line  to  the  middle  of  the  field.  The  spec- 
trum will  be  divided  lengthwise  into  two  halves,  one 
very  bright — the  spectrum  of  the  sun  itself — the  other 
fainter — the  spectrum  of  our  own  illuminated  atmos- 
phere. Focus  the  view-telescope  very  carefully  upon 
the  dust-lines  in  the  spectrum  (which  are  always  more 
than  sufficiently  abundant),  and  see  if  then  the  dividing 
boundary  between  the  two  halves  of  the  spectrum  is 
also  perfectly  sharp.  If  not,  correct  the  approximate 
adjustment  slightly  until  this  condition  is  fulfilled. 
This  last  adjustment  can  be  made  satisfactorily  only 


ADJUSTMENT   OF  SPECTROSCOPE.  355 

when  the  air  is  very  steady,  and,  once  obtained,  per- 
manent marks  should  be  made  which  will  enable  it  to 
be  recovered  if  disturbed.  Moreover,  the  adjustment 
will  be  different  for  the  different  rays  of  ,the  spectrum 
(see  page  73).  The  focal  length  of  an  ordinary  achro- 
matic object-glass  is  shortest  for  rays  between  D  and  E, 
is  a  little  longer  for  C  and  F,  and  about  the  same  for 
both,  and  is  considerably  longer  for  the  indigo  and 
violet.  In  a  telescope  with  a  focal  length  of  a  dozen 
feet  the  difference  between  E  and  II  is  fully  three- 
quarters  of  an  inch. 

It  is  perhaps  worth  noting  that  the  best  thing  with 
which  to  clear  out  dust  from  the  slit,  so  far  as  the 
author's  experience  goes,  is  a  little  piece  of  soft  white 
pine  shaped  like  a  toothpick ;  almost  any  other  kind  of 
wood  leaves  shreds  in  the  slit-jaws. 

NOTE  B. 

Supplementary  to  page  81. 

As  a  rule,  the  spectrum  of  a  luminous  gas  consists 
of  a  great  number  of  lines,  sometimes,  as  in  the  case  of 
iron  vapor,  to-  be  counted  by  thousands. 

In  the  spectra  of  many  of  the  elements  there  is  no 
obvious  regularity  in  the  arrangement  and  spacing  of 
the  lines ;  but  in  many  also  the  lines,  as  has  been  shown 
by  Balmer,  Kayser  and  Runge,  and  other  observers, 
fall  more  or  less  completely  into  "series,"  in  which 
they  are  spaced  with  perfect  regularity  according  to 
very  simple  mathematical  formulae. 

The  lines  of  hydrogen,  for  instance,  form  a  single 
series :  beginning  with  the  C-line  in  the  red,  we  have 
next  F  in  the  blue,  and  this  is  followed  by  some  twenty 
more  (already  observed  in  the  laboratory  and  in  the 


356  SUPPLEMENTARY  NOTES. 

spectra  of  the  sun  and  various  stars)  which  crowd  more 
and  more  closely  together  as  they  run  up  into  the  ultra- 
violet region  of  the  spectrum,  their  wave-lengths  being  all 

given  by  the  one  simple  formula,  X,  =  3646'!  {  — , —  J, 

in  which  \  is  the  wave-length  of  a  line  on  Rowland's 
scale,  and  n  is  always  one  of  the  even  numbers,  6,  8,  10, 
12,  etc.,  beginning  with  6,  since  4  or  any  smaller  num- 
ber would  give  an  impossible  result. 

It  is  a  very  curious  fact  that,  while  ordinarily  we 
find  in  the  spectrum  of  hydrogen  only  lines  that  corre- 
spond to  the  even  values  of  n,  Pickering  has  found  in 
the  spectra  of  certain  stars,  of  which  Zeta  Puppis  is 
the  most  conspicuous  example,  a  series  of  intermediate 
lines  which  fall  in  between  the  lines  of  the  usual  hydro- 
gen series  and  correspond  accurately  to  the  same  for- 
mula with  the  odd  values  of  n  (5,  7,  9,  11,  etc.),  replac- 
ing the  even. 

More  frequently  than  otherwise  the  spectrum  of  a 
substance  contains  more  than  one  series — that  of  helium, 
for  instance,  consists  of  six.  In  some  cases  the  series  run 
in  the  opposite  direction,  the  successive  lines  crowding 
more  and  more  closely  together  as  they  approach  the  red 
end  of  the  spectrum ;  this  is  the  case  with  the  hydrocar- 
bon bands  which  characterize  the  spectrum  of  a  comet. 

Yery  often,  perhaps  usually,  the  spectrum  of  an 
element  contains  besides  the  lines  which  fall  into  series 
a  number  of  others  which  seem  to  follow  no  law ;  and 
not  unf requently  these  unsocial  lines  are  among  the 
most  important  and  conspicuous  of  all ;  for  instance, 
the  H  and  K  lines  of  calcium. 

The  explanation  of  these  peculiarities  of  spectrum 
structure  is  not  yet  known,  but  must  be  connected  with 
the  structure  of  the  molecule  itself. 


SPECTRUM   OF   THE   CORONA.  357 


NOTE  C. 

ON   THE   REVERSING    STRATUM   AND   ITS    SPECTRUM,    AND 
ON   THE    SPECTRUM   OF   THE   CORONA. 

Supplementary  to  pages  83,  325,  and  339,  and  to  pages  258-262. 

The  observation  made  by  the  writer  in  1870,  de- 
scribed on  pages  82  and  83,  received  a  beautiful  photo- 
graphic confirmation  during  the  total  eclipse  of  1896. 
Mr.  Shackleton,  the  photographer  of  an  English  party 
at  a  station  in  Nova  Zembla  (the  only  party  which  was 
not  baffled  by  bad  weather),  secured  an  instantaneous 
photograph  at  the  critical  moment  with  a  so-called  "  pris- 
matic camera,"  which  is  simply  a  camera  with  (in  this 
case)  two  large  prisms  in  front  of  its  lens,  no  collimator 
being  used — a  photographic  "  slitless  spectroscope." 

When  the  sun's  disk  is  reduced  to  an  extremely 
narrow  crescent  by  the  encroaching  moon,  this  crescent 
itself  acts  like  the  slit  of  an  ordinary  spectroscope,  and 
photographs  taken  with  such  an  instrument  immediately 
before  totality  are  just  like  the  usual  solar  spectrum, 
except  that  the  dark  Fraunhofer  lines  are  replaced  by 
dark  crescents — negative  images,  so  to  speak,  of  the 
still  uncovered  portion  of  the  disk.  As  soon,  however, 
as  the  photosphere  disappears,  the  remaining,  much 
fainter,  crescent  is  simply  the  solar  atmosphere,  and  if 
the  observation  of  1870  was  correct,  its  photograph 
ought  to  show  a  series  of  bright  images  replacing  the 
former  dark  ones,  and  it  did. 

Mr.  Shackleton  watched  the  waning  crescent  with  a 
small  direct-vision  prism  held  in  the  hand,  and  at  the 
instant  when  the  brilliant  dark-lined  spectrum  vanished 
he  "pressed  the  button"  and  caught  on  his  plate  the 
"  flash-spectrum,"  as  it  has  been  called  by  Mr.  Lockyer. 


358  SUPPLEMENTARY  NOTES. 

The  exposure  was  about  half  a  second.  The  photo- 
graph shows  a  long  range  of  several  hundred  bright, 
curved  images,  of  which  there  are  nearly  250  in  the  blue 
portion  of  the  spectrum  between  F  and  H.  About 
25  are  much  more  extensive  and  conspicuous  than 
the  others,  and  are  images  of  the  chromosphere  and 
prominences.  They  are  due  to  hydrogen,  calcium, 
helium,  strontium,  and  one  or  two  other  elements 
which  often  appear  in  the  chromosphere.  The  rest 
are  simply  reversals  of  the  Fraunhofer  lines,  as  Mr. 
Shackleton  has  shown  by  developing  the  flash-spectrum 
into  a  bright-line  spectrum  of  the  usual  form  (which  is 
easily  done  by  a  simple  mechanical  contrivance),  and 
comparing  it  with  an  ordinary  dark-line  solar  spectrum 
photographed  with  the  same  camera  and  prisms,  but 
with  the  addition  of  a  collimator  and  slit.  The  agree- 
ment is  practically  complete,  although  there  are  two  or 
three  somewhat  conspicuous  Fraunhofer  lines  which  are 
missing  in  the  flash-spectrum,  probably  because  they 
originate  not  above  the  surface  of  the  photosphere, 
but  in  its  depths,  as  probably  also  do  the  wide  hazy 
shadings  that  accompany  the  H  and  K  lines  and  some 
others,  but  this  is  a  matter  for  further  investiga- 
tion. 

A  second  photograph,  taken  not  more  than  five  or 
six  seconds  later,  shows  only  the  chromospheric  images, 
proving  of  course  that  the  stratum  of  the  solar  atmos- 
phere which  produces  the  Fraunhofer  lines  by  its  ab- 
sorption must  be  extremely  thin.  This  is  perfectly  in 
accordance  with  the  view  expressed  on  pages  325  and 
339,  and  does  not  at  all  favor  the  opposite  "Dissociation 
Theory  "  of  Mr.  Lockyer,  according  to  which  the  lines, 
many  of  them  at  least,  are  produced  only  at  a  consid- 
erable elevation,  where  the  temperature  is  low  enough 


SPECTRUM   OF  THE   CORONA.  359 

to  allow  the  recombination  of  elements  dissociated  in 
the  hotter  regions  underneath. 

A  photograph  made  by  the  same  instrument  about 
the  middle  of  the  eclipse,  with  an  exposure  of  nearly  a 
minute,  shows  Very  finely  the  green  coronal  ring,  corre- 
sponding to  the  old  "  1,474  line  "  and  several  others  in 
addition.  These  are  all  in  the  violet  part  of  the  spec- 
trum, and  are  extremely  faint,  excepting  one  which  is 
a  little  below  H.  They  are  all  probably  due  to  the 
same  hypothetical  element,  still  unidentified,  but  pro- 
visionally named  "coronium."  The  photograph  also 
seems  to  make  it  certain  that  hydrogen,  helium,  and 
calcium,  though  brilliantly  conspicuous  upon  the  plate 
in  the  images  of  the  prominences,  are  entirely  absent 
from  the  cwona,  a  result  agreeing  with  that  deduced 
from  similar  photographs  made  in  1893,  but  only  re- 
cently published.  It  is  quite  clear  that  the  earlier  ob- 
servations (referred  to  on  pages  260,  261,  and  262) 
were  misleading  from  the  fact  that  the  apparatus  did 
not  sufficiently  guard  against  the  effects  of  the  illumina- 
tion of  the  air  by  light  from  the  prominences. 


NOTE  D. 

SHIFT   OF    SPECTRUM    LINES    CAUSED   BY   CHANGE   OF 
PRESSURE. 

Supplementary  to  page  98. 

An  important  series  of  observations,  made  at  the 
Johns  Hopkins  University  by  Humphreys  and  Mohler 
in  1895-'96,  has  shown  conclusively  that  changes  of 
pressure  alter  the  position  of  lines  in  the  spectrum  in 
precisely  the  same  way  as  does  motion  in  the  line  of  sight 
(approach  or  recession). 


360  SUPPLEMENTARY   NOTES. 

Increase  of  pressure  upon  gaseous  molecules  which 
are  emitting  or  absorbing  light  shifts  the  lines  of  the 
spectrum  toward  the  red  by  an  amount  which  is  pro- 
portional to  the  change  of  pressure  and  to  the  wave- 
length of  the  line  affected,  but  is  different  for  different 
elements,  and,  in  some  cases  at  least,  differs  for  the 
lines  belonging  to  the  different  "  series  "  (Note  B,  page 
355)  in  the  spectrum  of  a  given  element ;  thus,  for  in- 
stance, the  shift  of  H  and  K  is  only  half  as  great  as 
that  of  the  calcium-lines  which  belong  to  the  two  regu- 
lar series. 

This  pressure  effect  is  very  slight ;  in  the  case  of 
most  substances  a  pressure  change  of  ten  atmospheres 
(150  pounds  to  the  square  inch)  produces  a  displace- 
ment much  less  than  would  a  velocity  of  a  mile  a  sec- 
ond. Still,  even  this  amount  is  a  quantity  which  can 
not  be  neglected  in  the  investigation  of  the  motions  of 
the  stars,  and  the  comparison  of  the  difference  of  the 
shift  of  lines  of  different  elements  may  ultimately  give 
us  important  information  as  to  the  atmospheric  pres- 
sures existing  upon  the  stellar  surfaces,  and  so  of  the 
intensity  of  gravity  there  prevailing.  In  the  "  revers- 
ing layer"  of  the  sun  the  pressure  appears  to  be  be- 
tween four  and  seven  atmospheres. 


NOTE  E. 

Supplementary  to  pages  141-144. 

Kecent  important  and  elaborate  mathematical  in- 
vestigations by  Wilsing,  of  Potsdam,  and  Sampson,  of 
Durham  College,  England,  point  to  the  conclusion  that 
the  explanation  of  the  "  Equatorial  Acceleration  "  of  the 
sun,  which  has  so  long  perplexed  the  students  of  solar 


EQUATORIAL  ACCELERATION.  361 

physics,  is  to  be  sought  not  in  forces  and  conditions  at 
present  acting,  but  in  a  slowly  dying  "  survival  "  from 
an  earlier  state  of  things.  As  Wilsing  expresses  it, 
"  We  are  relieved  from  the  difficulty  of  accounting  for 
the  maintenance  of  these  currents  on  mechanical  and 
physical  principles,  since  they  appear  as  the  result  of 
earlier  conditions  of  motion."  While  the  present  con- 
ditions are  such  as  tend  to  destroy  the  observed  in- 
equalities of  motion,  and  will  ultimately  bring  the  sun 
to  the  same  state  of  uniform  rotation  as  a  solid  globe, 
yet  the  process  is  so  slow  that  the  present  behavior  will 
continue  practically  unchanged  for  thousands,  if  not  for 
millions,  of  years.  Near  the  center  of  the  sun  this 
state  of  uniform  rotation  has  probably  been  already 
reached,  but  it  will  be  ages  before  the  surface  drifts 
will  disappear,  and  the  change  in  a  century  or  two  will 
be  quite  too  small  to  be  detected  by  any  possible  obser- 
vations. 

It  is  perhaps  worth  while  to  add  in  this  connection 
that  the  spectroscopic  observations  of  Jewell  and  others 
at  Johns  Hopkins  give  results  as  to  the  sun's  rotation  in 
practical  agreement  with  those  of  Duner,  quoted  on  page 
101.  They  appear  also  to  indicate  further  that  the  lower 
lying  strata  of  the  solar  atmosphere  move  more  slowly 
than  those  at  a  higher  elevation,  and  take  longer  to 
complete  a  rotation.  But  the  observations  are  so  deli- 
cate that  this  can  not  yet  be  considered  as  established 
beyond  question. 


INDEX. 


ABBE,  extent  of  corona  in  eclipse 
of  1878,  246. 

Absorption  of  sun's  rays  by  the  at- 
mosphere of  the  earth,  297,  298. 

Actinic  or  chemical  rays,  285. 

Adjustment  of  focal  plane  of  tele- 
scope to  the  slit  of  the  spectro- 
scope for  observations  upon  the 
spectrum  of  the  chromosphere, 
207. 

slit  of  spectroscope  for  ob- 
servations of  the  prominences, 
210,  211. 

Age  and  duration  of  the  sun,  816- 
318. 

Airy,  solar  parallax  from  the  transit 
of  1874,  24. 

Allotropic  states  of  chemical  ele- 
ments, 91. 

American  method  of  photographing 
the  transit  of  Venus,  28,  29. 

Analyzing  spectroscope,  73. 

Andrews,  critical  temperature  of  a 
gas,  329. 

Angstrom,  A.  J.,  early  studies  in 
spectrum  analysis,  60,  80. 

—  map  of  solar  spectrum,  78,  92,  93. 

Angstrom,  K.,  measures  of  solar 
radiation,  304. 

Animal,  body  of,  regarded  as  a  ma- 
chine, 3,  4. 

Arago,  diminution  of  brightness  at 
the  limb  of  the  sun,  279. 

Aristarchus,  method  of  determining 
the  sun's  parallax,  14. 


"Arrow-head"  appearance  of  hy- 
drogen lines  in  chromosphere 
spectrum,  209,  210. 

Ascension  Island,  19. 

Asteroids,  observed  for  solar  paral- 
lax, 21. 

Aurora  boreal  is,  its  spectrum  not 
to  be  identified  with  that  of  the 
corona,  260. 

relation  to  sun-spots,  164, 165. 

resemblance  between  its 

streamers  and  those  of  the  co- 
rona, 265,  273. 

Axis  of  the  sun,  144-146. 

table  giving  its  posi- 
tion-angle for  different  times  of 
the  year,  146. 

BALL,  Sir  EGBERT,  on  carbon 
in  sun,  339. 

Basic  lines  in  solar  spectrum,  92,  93. 

Barker,  dark  lines  in  spectrum  of 
the  corona,  261. 

Belli,  photometric  observation  upon 
the  brightness  of  the  corona,  254. 

Belopolsky,  rotation  of  sun,  143. 

Bessemer  converter,  compared  with 
the  solar  radiation,  279,  324. 

Biela,  brightness  of  the  inner  corona, 
255. 

Bigelow,  F.  H.,  on  sun's  rotation 
period,  137. 

theory  of  corona,  273. 

Blueness  of  sunlight  before  suffer- 
ing atmospheric  absorption,  284. 


363 


364 


INDEX. 


Bolometer  described,  299. 

—  determination  of  true   value    of 
the  solar  constant,  298. 

—  sensitiveness  of,  299. 
Bouguer,  measurement  of  the  sun's 

light,  275. 

Brester,  theory  of  prominences,  228. 

Brightness  of  the  corona,  253-256. 

Bullock,  drawing  of  eclipse  of  1868, 
244. 

Bunsen,  arrangement  of  spectro- 
scope scale,  63. 

—  work  upon  the  solar  spectrum  in 
connection  with  Kirchhoff,  60. 

Burning-glass,  effect  of,  308,  309. 

CLINE,  in  chromosphere  and 
prominences,  197,  206,  209,  210, 
232. 

photograph  of  prominence,  232. 

, photograph  of  its  double  re- 
versal, 209. 

Candle-power,  or  photometric  unit, 
defined,  275. 

Calcium-light  compared  with  sun- 
light, 278,  324. 

Calcium  lines  in  spectrum,  see  H  and 
K  lines. 

Calory,  or  thermal  unit,  defined,  289. 

Capocci,  theory  that  spots  are  due 
to  volcanic  eruptions  on  the  sun, 
178. 

Carbon,  as  chief  constituent  of  pho- 
tosphere, 339. 

Carrington,  discovery  of  sun's  equa- 
torial acceleration  and  formula 
for  it,  138-140. 

—  distribution  of  sun-spots,  147, 148. 

—  method  of  determining  the  posi- 
tion of  a  spot  on  the  sun,  45. 

—  motion  of  spots  in  latitude,  146. 

—  observation  of  remarkable  solar 
outburst,  November  1, 1859, 120. 

—  period  of  sun's  rotation,  137, 138. 

—  position  of  sun's  axis,  144. 
Cassini,  observations  for  solar  par- 
allax, 18. 


Christie,  solar  eyepiece,  58. 
Chromosphere  defined,  7, 192. 
Cleveite,  mineral  from  which  helium 

is  obtained,  345,  346-348. 
Coal,  consumption  of  which  would 

be  required  to  keep  up  the  solar 

radiation,  289. 
Comets'  tails,  their  analogies  to  th« 

streamers  of  the  corona,  265,  342. 
Comparison-prism,  84. 
Concave-grating  spectroscope,  70. 
Condensation  theory  of  solar  heat, 

314-316. 
Constancy  of  solar  heat  during  the 

historic  period,  311. 
Constitution  of  sun,  8,  325-335. 
Contact  observations  at  the  transit 

of  Venus,  23,  24. 

by  means  of  photography,  26. 

Corona,  brightness  of,  253-256. 

—  defined,  7. 

—  examined  by  slitless  spectroscope, 
261,  262. 

Corona-line  in  the  spectrum,  discov- 
ery, 250. 

duplicity  of,  257. 

map,  258. 

not  identical  with  line  in  spec- 
trum of  aurora  borealis,  260. 

Cornu,  solar  photography,  47. 

Crew,  H.,  spectroscopic  determina- 
tion of  the  sun's  rotation  period, 
100. 

Critical  temperature  of  a  gas,  329. 

Croll,  hypothesis  that  a  portion  of 
the  sun's  energy  may  have  origi- 
nated in  a  collision  with  a  star, 
318. 

Crookes,  spectrum  of  helium,  346. 

Crova,  pyrheliometer,  292. 

—  value  of  solar  constant,  296. 
Cyclonic  motion  in  sun-spots,  126, 

183,184. 

DLINE     of     helium,    88,    198, 
*     220,  345-348. 
dark  in  sun-spot  spectrum,  135. 


INDEX. 


365 


D  lines  of  sodium,  doubly  reversed, 
208. 

Dalton,  law  of  gaseous  mixture,  332, 
333. 

Dark  lines  in  the  solar  spectrum  dis- 
covered, 59. 

explanation  of,  80,  81-. 

in  spectrum  of  the  corona,  261. 

Dartmouth  College  spectroscope,  73, 
74. 

Davis,  photograph  of  eclipse  of  1871, 
248. 

Dawes,  "  holes  "  in  nucleus  of  sun- 
spot,  118. 

—  solar  eyepiece,  58. 

Dehra  Dun,  photographs  of  sun,  52. 

De  La  Kue,  the  Kew  photohelio- 
graph,  48,  49. 

photographs  of  the  eclipse 

of  1860, 195. 

measures  of  sun-spot  pe- 
numbra, 130. 

planetary  influence  on  sun- 
spot  development,  158. 

—  relation  of  Wolfs  "relative 
numbers"  to  the  spotted  area  of 
the  sun,  154. 

Denza,  bright  lines  in  corona  spec- 
trum, 260. 

Derham,  volcanic  theory  of  sun- 
spots,  178. 

Deslandres,  on  spectrum  of  faculae, 
109. 

spectro-heliographic  work,  236. 

Detached  cloud-formed  prominences 
and  their  development,  221-222. 

Development  of  sun-spots,  122. 

Deville,  estimate  of  the  temperature 
of  the  sun,  305. 

Diameter  and  dimensions  of  the  sun, 
38,  323. 

illustrations,  39. 

Diffraction  grating,  66,  70. 

—  spectroscope,  68-70. 

—  spectrum,  72. 

Dimensions  of  sun-spots,  127, 128. 
Diminution  of  brightness  at  limb  of 


the  sun,  45,  108,  279-284,  324, 
337. 

Discovery  of  bright  line  in  corona 
spectrum,  250. 

dark  lines  in  solar  spectrum,  59. 

dark  lines  in  spectrum  of  co- 
rona, 262. 

elements  present  in  the  sun, 

86-88. 

equatorial  acceleration  of  the 

sun,  138-140. 

explanation  of  cause  of  dark 

lines,  80. 

gaseous  constitution  of  the 

prominences,  197. 

magnetic  relations  of  sun- 
spots,  162. 

periodicity  of  sun-spots,  151. 

reversing  layer  of  the  sun,  82. 

spectroscopic  method  of  ob- 
serving prominences,  198. 

sun-spots,  114. 

terrestrial  helium,  345-348. 

Displacement  and  distortion  of  lines 
by  motion,  97-101,  208. 

Dissolution  and  disappearance  of 
sun-spots,  122-124. 

Distances  (relative)  of  planets,  16. 

Distance  of  the  sun  from  the  earth, 
illustrations,  36,  37. 

Distortion  of  forms  of  prominences, 
by  spectroscope,  205. 

Distribution  of  sun-spots  and  prom- 
inences in  solar  latitude,  147,  148, 
214. 

Don  Ulloa,  observation  of  "hole  in 
the  moon,"  in  the  eclipse  of  1778, 
194. 

Doppler's  principle,  97-98. 

Draper,  Dr.  Henry,  oxygen  in  the 
sun,  94,  95. 

Draper,  J.  W.,  early  spectroscopic 
researches,  60. 

Drawings  of  corona,  discrepancies, 
239. 

Dulong  and  Petit,  law  of  radiation, 
306,  heats,  92. 


366 


INDEX. 


Duner,  spectroscopic  determination 
of  the  sun's  rotation  period,  101. 

on  sun-spot  spectrum,  132,  339. 

Duration  of  sun-spots,  119. 

EARTH,  dimensions  of  the,  12. 
—  her  share  of  the  solar  radi- 
ation, 290. 

.  Eastman,  photometric  observations 
during  eclipse  of  1869,  253. 

Eclipse,  solar,  1706—194;  1715— 
194;  1733—193;  1778—194;  1806 
—194;  1842—194,254;  1851—195; 
1857—240 ;  1860—195, 241, 242, 256 ; 
1867—243;  1868—196,  197,  244; 
1869—245,  250, 260 ;  1870—83 ;  1871 
—246-248, 256,  265 ;  1878—239,  249, 
264;  1882—251 ;  1889—253  ;  1893— 
255. 

general  phenomena,  237,  238. 

Ecliptic,  denned,  6. 

Effect  of  changes  in  solar  atmos- 
phere upon  terrestrial  conditions, 

.    303. 

Effective  temperature  of  the  sun, 
305. 

Egoroff,  identifies  A  and  B  lines  of 
solar  spectrum  as  due  to  oxygen, 
93. 

Electric  light  compared  with  sun- 
light, 278,  324. 

Elements  known  to  be  present  in 
the  sun,  table,  87,  88. 

Encke,  discussion  of  transits  of 
Venus  in  1761  and  17(59,  22,  36. 

Energy  (total)  of  solar  radiation, 
288-290. 

Energy,  terrestrial,  mainly  derived 
from  solar  heat,  2,  3. 

—  —  other  sources  than  solar  heat,  4. 
Equatorial  acceleration  of  the  sun, 

138-140. 

explanation  suggested,  141-144. 

Ericsson,  estimate  of  the  sun's  tem- 
perature, 305. 

—  experiment   upon    radiation    of 
molten  iron,  309. 


Ericsson,  measure  of  solar  heat,  293. 

—  solar  engine,  290. 

Eruptive  prominences,  218-223. 

Experiment,  showing  that  the  black- 
ness of  the  dark  lines  in  the  spec- 
trum is  only  relative,  80,  81. 

Explanation  of  the  sun's  eruptive 
action  caused  by  the  constriction 
of  the  photosphere,  227. 

FABEICIUS,    discovery    of   sun- 
spots,  114. 

Faculse.  106-108. 
-  —  spectrum  of,  109. 

identical  or  not  with  promi- 
nences, 109. 

used  in  determining  sun's  ro- 
tation period,  140. 

Faye,  explanation  of  the  sun's  equa- 
torial acceleration,  143. 

—  formula  for  the  acceleration,  139. 

—  theories  of  sun-spots,  181,  182- 
185. 

—  computation    of   solar    parallax, 
23. 

Ferrers,  observation  of  eclipse  of 
1806, 192. 

Fizeau,  comparison  of  electric  and 
calcium  light  with  sunlight,  278. 

Flamsteed,  method  of  deducing  so- 
lar parallax  from  observations  of 
Mars,  15, 19. 

Flashes  reported  by  Peters  in  sun- 
spots,  123. 

Foenander,  drawing  of  eclipse  of 
1871,  247. 

Forbes,  value  of  solar  constant,  296. 

Foucault,  comparison  of  electric  and 
calcium  lights  with  sunlight,  278. 

—  determination  of  the  velocity  of 
light,  34. 

—  early  spectroscopic  researches,  60. 
Fourteen  hundred  and  seventy-four 

line,  257-260,  262. 
Frankland  names  helium,  344. 
Fraunhofer,  discovery  of  dark  lines 

in  solar  spectrum,  59. 


INDEX. 


367 


Fraunhofer,  coincidence  of  D  line  in 
solar  spectrum  with  bright  line  in 
flame  spectrum,  80. 

—  map  of  the  spectrum,  77. 

—  use  of  a  prism  to  separate  overly- 
ing diffraction  spectra,  68. 

Frost,  E.  B.,  temperature  of  a  sun- 
spot,  170. 

—  solar  radiation,  302. 

pALILEO,  discovery  of  sun-spots, 
VI     114. 

—  theory  of  sun-spots,  178. 

Gas,  Dalton's  law  of  mixture,  332, 
333. 

—  distinctive  properties,  332,  333. 

—  Lane's  law  of  temperature  and 
condensation,  315. 

—  liquefaction  and  critical  tempera- 
ture, 329. 

—  viscosity  at  high  temperatures, 
331. 

Gaseous  condition  of  the  sun's  nu- 
cleus, 327-332. 

Gautier,  relation  between  magnet- 
ism and  sun-spots,  163. 

Gill,  observations  of  Mars  for  deter- 
mination of  solar  parallax,  19-21. 

—  observations  of  asteroids,  21. 
Gilman,  corona  of  eclipse  of  1869, 

244. 

Gould,  diminution  of  the  earth's 
temperature  at  sun-spot  maximum, 
172. 

Granulation  of  the  sun's  surface, 
102-105. 

Grant,  early  recognition  of  the  chro- 
mosphere, 195,  200. 

Grating,  diffraction,  used  in  spectro- 
scope, 66-71,  204. 

concave,  70. 

Gray,  Wilson  and,  on  temperature 
of  sun,  307. 

Greenwich  magnetic  record  for  Au- 
gust 3  and  5,  1872,  168. 

—  solar  photographs,  52. 
Gregory  first  calls  attention  to  tran- 


sits of  Venus  as  a  means  of  deter- 
mining the  sun's  parallax,  22. 
Grosch,  drawing  of  eclipse  of  1867, 
243. 

HL1NE  in  spectrum  of  corona, 
258,  260. 

in  spectrum  of  faculee,  109. 

reversal  in  prominences,  206, 

231-233. 

double  reversal,  109,  231. 

reversed  in  the  spectrum  of 

sun-spots,  109,  134. 
Habitability  of  the  sun,  179, 330. 
Hale,  G.  E.,  on  spectrum  of  faculse, 

109. 

—  —    spectro-heliographic    work, 
234,  235. 

—  recognition  of  duplicity  of  D3  in 
chromosphere  spectrum,  348. 

Halley,  determination  of  the  sun's 
parallax  by  transit  of  Venus,  22. 

Hansen,  detection  of  error  in  the  re- 
ceived value  of  the  sun's  parallax, 
23,  36. 

Harkness,  observation  of  the  bright 
line  in  the  corona  spectrum,  250. 

Hastings,  comparison  of  the  spec- 
trum of  the  sun's  limb  with  that 
of  the  central  portion  of  the  disk, 
83. 

—  smoke-like  nature  of  the  layer 
which  causes  the  darkening  of  the 
sun's  limb,  282,  337. 

—  theory  of  the  constitution  of  the 
sun,  336-340. 

Heat  derived  from  stars  and  meteors, 
4. 

Heliometer  described,  19. 

Helioscopes  and  helioscopic  eye- 
pieces, 54-57. 

Helium  and  its  characteristic  lines, 
88,  260. 

—  identification  as  a  terrestrial  ele- 
ment by  Ramsay,  88,  345. 

—  in  the  spectrum  of  the   promi- 
nences, 206,  207,  220,  348. 


368 


INDEX. 


Helium,  note  on  its  discovery  and 
properties,  344-350. 

Helmholtz,  contraction  theory  of  the 
solar  heat,  315. 

Henry,  observations  with  the  ther- 
mopile upon  radiation  of  sun-spots 
and  different  portions  of  the  sun's 
disk,  170,  298. 

Herschel,  Sir  John,  measurement  of 
the  sun's  heat,  286-289. 

meteors  as  the  cause  of  the 

sun's  equatorial  acceleration,  141. 

solar  eyepiece,  55. 

theory  of  sun-spots,  179. 

Herschel,  Capt.  John,  observation  of 
prominence  spectrum  in  1868, 197. 

Herschel,  Sir  W.,  relation  between 
sun-spots  and  price  of  wheat,  152. 

—  theory  of  the  sun-spots  and  the 
sun's  constitution,  179. 

Higgs,  G.,  photographic  maps  of 
solar  spectrum,  79. 

Hillebrand,  first  discovers  gas  in 
cleveite,  345. 

Hodgson,  observation  of  solar  out- 
burst in  1859, 120. 

Horrebow,  anticipation  of  the  peri- 
odicity of  sun-spots,  152. 

Hewlett,  views  on  nature  of  sun- 
spots,  129. 

Huggins,  granulation  of  the  sun's 
surface,  105. 

—  use  of  widened  slit  in  observing 
forms  of  prominences,  201. 

Hydrogen-lines  in  the  spectrum  of 
the  corona,  258. 

ICE,    quantity    which    would    be 
melted  in  a  minute  by  the  sun's 
radiation,  288,  289. 
Intra-Mercurial  planet,  314. 
Investigation  as  to  the  influence  of 
the  planets  upon  the  generation  of 
sun-spots,  158, 159. 

TANSSEN,  discovery  of  method  of 
*J    observing  prominences  by  means 
of  the  spectroscope,  196, 197. 


Janssen,  medal  from  French  Gov- 
ernment, 200. 

—  observation  of  the  eclipse  of  1868, 
198,  344. 

—  observations  of  the  eclipse  of  1871 
and  recognition  of  bright  lines  ot 
hydrogen    and    dark    Fraunhofer 
lines  in  the  corona  spectrum,  258. 

—  observation  of  Venus  on  the  co- 
rona, 229. 

—  photographic  contact  at  the  tran- 
sit of  Venus,  26. 

—  Reseau  Photospherique,  112. 

—  shows  A  and  B  lines  to  be  atmos 
pheric,  93. 

—  solar  photography,  52, 112. 
Jelinek,  influence  of  sun-spots  on 

the  temperature  of  the  earth,  172. 

Jevons,  connection  between  sun- 
spots  and  commercial  crises,  176, 
177. 

Jukowsky,  rotation  of  sun,  143. 

Jupiter,  influence  upon  sun-spots, 
158, 159. 

—  satellites  of,  observed  to  determine 
the  equation  of  light,  34,  35. 

KLINE,   its  reversal   in  faculee, 
109. 

reversal  in  sun-spots,  109. 

reversal  in  chromosphere  and 

prominences,  109. 

double  reversal  of,  231. 

in  spectro-heliographic  work, 

234,  235. 
Kelvin,  Lord,    endurance   of  sun's 

heat  if  produced  by  the  combustion 

of  coal,  290. 
estimate  of  heat  which  would  be 

produced  by  fall  of  planets  upon 

the  sun,  312,  313. 
on  connection  between  sun-spots 

and  magnetic  storms,  169. 
Kew.  photoheliograph,  and  photo- 
graphic record,  48,  49. 
Kirchhoff,  map  of  solar  spectrum, 

78, 184,  225,  257. 


INDEX. 


369 


Kirchhoff,  spectroscopic  work,  60, 80, 
81,  86. 

—  theory  of  sun-spots,  178. 

T  ACAILLE,  observations  for  solar 

-U    parallax,  18. 

Lalande,  theory  of  sun-spots,  178. 

Lambert,  diminution  of  light  at  the 
limb  of  the  sun,  279. 

Lane,  estimate  of  the  sun's  tempera- 
ture, 305. 

—  law  relating  to  the  temperature  of 
a  contracting  mass  of  gas,  315. 

Langley,  bolometer  and  bolometric 
observations,  296-302. 

—  color  of  the  sun's  limb  compared 
with  that  of  the  center  of  the  disk, 
282. 

—  comparison  between  the  intensity 
of  solar  radiation  and  that  of  metal 
in  a  Bessemer  converter,  279,  309. 

—  details  of  the  solar  surface  (frontis- 
piece), 103. 

—  diminution  of  heat  at  the  sun's 
limb,  281. 

—  diminution  of  light  at  the  sun's 
limb,  302. 

—  effect  of  the  sun's  atmosphere  and 
its  changes  upon  the  earth's  tem- 
perature, 303. 

—  extent  of  corona  in  eclipse  of  1878, 
245,246. 

—  increase  of  solar  radiation  due  to 
disturbance  of  the  sun's  surface, 
170. 

—  observation  of   Mercury  at  the 
transit  in  1878,  255. 

—  solar  eyepiece,  58. 

—  spectroscopic  observation  of  the 
sun's  rotation,  100. 

—  temperature    of    sun-spots,    170, 
note. 

—  thermopile  observations,  302. 

—  true  color  of  the  sun,  284. 
Laplace,  effect  of  the  absorption  of 

the  atmosphere  of  the  sun  upon 
its  brightness.  283. 
25 


Latitude  of  sun-spots,  Spoerer's  dis- 
covery, 157. 

Laugier,  sun's  equatorial  accelera- 
tion, 138. 

Laussedat,  horizontal  photohelio- 
graph,  27. 

Le  Chatelier,  on  the  temperature  of 
the  sun,  307. 

Lens, -burning  effect  of,  308,  309. 

Leverrier,  determination  of  the  par- 
allax of  the  sun  by  means  of 
planetary  perturbations,  16. 

—  perturbations  of  Mercury  indicat- 
ing intra-Mercurial  planets,  314. 

Liais,  drawing  of  eclipse  of  1857, 

240. 
Light  of  the  sun,  its  total  quantity 

in  standard  candle-power,  274, 324. 

—  its  intensity,  278,  324. 
method     of    measuring, 

266,  267. 

—  velocity  of,  used  in  determining 
the  solar  parallax,  16,  34,  35. 

Lindsay,  Lord,  expedition  to  Ascen- 
sion Island,  19. 

Liquefaction  of  gases,  329,  349. 

Lockyer,  arrangement  for  studying 
the  solar  spectrum,  83. 

—  connection  between  sun-spots  and 
rainfall  in  India  and  Africa,  175. 

—  discovery    of    the    spectroscopic 
method  of  observing  the  chromo- 
sphere and  prominences,  198-200. 

—  discovery  of  the  1,474  line  in  the 
chromosphere  spectrum,  257. 

—  medal  from  the  French  Govern- 
ment, 199. 

—  observation  of  the  lines  of  hy- 
drogen   in  the  corona  spectrum, 
258. 

—  theory  as  to  the  non-elementary 
character  of  so-called    elements, 
90,  92. 

—  use  of  annular  slit  for  observing 
circumference  of  the  sun,  213. 

—  vibrating  slit  for  observation  of 
prominences,  200. 


3YO 


INDEX. 


Loomis,  effect  of  conjunctions  of  J  u- 

piter  and    Saturn  upon  sun-spot 

periodicity,  159. 
• —  relation  between    sun-spots   and 

the  aurora  borealis,  164. 
Luminous    radiations,  groundlessly 

distinguished  from  thermal    and 

chemical,  285. 
Lunar  perturbations,  as  a  means  of 

determining  the  solar  parallax,  32. 

MAGNETISM,  terrestrial,  period 
of  disturbance  corresponding 
with  the  sun-spot  period,  162-164. 

aifected  by  solar  paroxysms, 

120, 121, 164-170. 

Mars,  observed  as  a  means  of  deter- 
mining solar  parallax,  17-21. 

Mass  of  the  sun,  39, 40,  323. 

Maunder,  on  relation  of  faculse  and 
prominences,  109. 

—  on  connection  between  sun-spots 
and    terrestrial     magnetism,    165, 
166. 

Maxwell,  effect  of  temperature  upon 
the  viscosity  of  a  gas,  331. 

Mechanical  equivalent  of  heat,  312. 

Medal  struck  by  the  French  Gov- 
ernment in  honor  of  Janssen  and 
Lockyer,  199. 

Meldrum,  connection  between  sun- 
spots,  cyclones,  and  rainfall,  173- 
175. 

Mercury  (planet),  influence  on  sun- 
spots,  158. 

—  perturbations    indicating    intra- 
Mercurial  matter,  314. 

—  seen  at  transit  on  the  background 
of  the  corona,  255. 

Merz  helioscope,  56. 
Metallic  prominences,  224. 
Metals,  present  in  the  sun,  87,  88. 
Meteors,  possibly  concerned  in  the 
formation  of  sun-spots,  160, 186. 

—  regarded  as  the  cause  of  the  sun's 
equatorial  acceleration,  141. 

Meteoric  iron,  helium  present  in,  346. 


Meteoric  theory  of  the  sun's  heat, 
312,  313. 

Meudon,  solar  observatory,  52,  111. 

Michelson,  determination  of  the  ve- 
locity of  light,  34. 

Mineral  waters,  helium  present  in, 
346. 

Motion  in  line  of  sight,  spectroscop- 
ically  observed,  97, 101. 

Mouchot,  solar  engine,  290. 

NASMYTH,  willow-leaf  structure 
of  the  sun's  surface,  104. 
Newcomb,  determination  of  the  so- 
lar parallax,  36. 

—  extent  of  the  corona  in  the  eclipse 
of  1878,  246. 

—  speculations    as  to  the  age  and 
duration  of  the  sun,  317,  318. 

Nodes  of  the  sun's  equator,  144. 

OLSZEWSK1,  attempt  to  liquefy 
helium,  349. 

Oppolzer,  E.,  sun-spot  theory,  190. 
Oxygen  in  the  sun,  Dr.  H.  Draper, 
94,  95. 

—  A  and  B  lines  of,  identified  by 
Egoroff,  93. 

shown  by  Janssen  to 

be  atmospheric,  93. 

—  spectra  of,  Schuster,  96. 

T)ALMIERI,  supposed  discovery  of 

A      helium  in  lava,  345. 

Parallax,  solar,  defined,  12. 

determined  by  lunar  perturba- 
tions, 32. 

determined  by  observations  of 

Mars,  17-21. 

determined  by  planetary  per- 
turbations, 32,  33. 

determined  by  transits  of  Ve- 
nus, 22-31. 

determined  by  the  velocity  of 

light,  34,  35. 

importance  and  difficulty  of 

the  problem,  10, 13. 


INDEX. 


371 


Parallax,  solar,  synopsis  of  methods 

for  its  determination,  15. 
values,  according  to  different 

authorities,  20,  21,  22,  25,  31,  32, 

35,  36,  323. 
Peters,    observations    of   sun-spots, 

123, 124. 
-  volcanic    theory    of   sun-spots, 

178. 
Petit,  observation  of  the  corona  in 

1860,  256. 

Photographic  observations  of  tran- 
sit of  Venus,  25-31. 
Photographs  of  eclipses,  1860—195; 

1871—241,    248,    250;    1882—246, 

251;    1889—247,    253;    1893—248, 

255. 

Photography,  solar,  47-53,  111,  112. 
Photoheliograph,  the,  48. 
Photometric  observations  upon  the 

corona,  252-256. 
Photosphere  defined,  7. 

—  theories  as  to  its  nature,  110, 143, 
182, 187, 188,  335-337. 

Picard,  observations  for  solar  paral- 
lax, 18. 

Pickering,  E.  C.,  diminution  of  light 
at  the  sun's  limb,  280,  283. 

—  effect    of  the    sun's  atmosphere 
upon  its  brightness,  284. 

—  solar  eyepiece,  58. 

Pitch  of  a  sound  altered  by  motion, 

98. 
Planetary  perturbations  as  a  means 

of  determining  solar  parallax,  32, 

33. 

—  influence    upon    sun-spots,    158- 
160. 

Planets,  determination  of  their  rela- 
tive distances,  16. 

Pogson,  observation  of  eclipse  of 
1868, 197. 

Polarization  of  the  corona,  261. 

Polarizing  eyepieces  or  helioscopes, 
57,  58. 

Position-angle  of  the  sun's  axis — 
table,  146. 


Potsdam,  astrophysical  observatory, 
308. 

Pouillet,  estimate  of  the  sun's  tem- 
perature, 305. 

—  measurement  of  the  sun's  heat, 
286,  291,  296. 

—  pyrheliometer,  291. 

—  temperature  of  space,  4. 
Powalky,  computation  of  solar  par- 
allax, 23. 

Princeton,  Henry's  thermopile  ob- 
servations, 170,  302. 

—  photographs  of  prominences  and 
their  spectrum,  209,  210,  231,  232. 

—  spectroscopes  used  in  the  observ- 
atory, 69,  76. 

Prisms  and  prismatic  spectrum,  61, 
65,  74. 

Problems  in  solar  physics,  342,  343. 

Proctor,  demonstration  that  the  co- 
rona can  not  be  due  to  the  earth's 
atmosphere,  250. 

—  velocity  of  matter  ejected  from 
the  sun,  227. 

Projection  of  the  sun's  image  on  a 
screen,  43. 

Prominences  or  protuberances  (so- 
lar), denned,  7. 

—  first  named,  192. 

Pupin,  M.  I.,  coronoidal  discharges, 
237. 

Purple  tint  of  the  nucleus  of  a  sun- 
spot,  118. 

Pyrheliometer,  291,  292. 

QUIESCENT    prominences,    218- 
221. 

pADIATION  (total)  of  the  sun, 

R>     288-290,  324. 

Ramsay,  identification  of  helium,  88, 

345. 
Kanyard,  brightness    of  the    inner 

corona,  255. 

—  memoir  on  recent  eclipses,  240. 

—  synclinal  structure  of  the  corona, 
264. 


372 


IXDEX. 


Kayet,  observation  of  the  eclipse  of 
1868,  197. 

—  discovery  of  helium  line,  x  7,065, 
344. 

Kayleigh,  Lord,  resolving  power  of 
spectroscopes,  66. 

—  discovery  of  Argon,  88,  345. 
Recurrence  of  sun-spots  at  special 

points  on  the  sun's  surface,  150, 
334,  335. 

Keed,  T.,  observation  of  duplicity  of 
D,,  348. 

—  photograph  of  prominence  in  C 
line,  232. 

Reflecting  telescope  with  unsilvered 
mirror  for  observing  the  sun,  54. 

Reseau  Photospherique,  Janssen, 
111,  112. 

Respighi,  depression  of  the  chromo- 
sphere over  a  sun-spot,  216. 

—  discovery  of  helium  line,  x  4,472, 
344. 

Reversal  of  bright  lines  to  dark  in 
the  solar  spectrum  explained, 
81. 

—  of  dark  lines  to  bright  in  a  total 
eclipse,  82. 

—  double,  of  C  line,  209,  210. 

—  double,  of  D  lines  in  the  chromo- 
sphere spectrum,  208. 

—  double,  of  H  and  K  lines,  109,  231. 
Reversing  stratum  of  the  solar  at- 
mosphere first  observed,  82,  83. 

its  relation  to  the  photosphere, 

325,  339. 

Richer,  observations  for  solar  paral- 
lax, 18. 

Roemer,  observations  for  solar  par- 
allax, 18. 

"  Rosa  Ursina,"  Scheiner,  115. 

Rosetti,  law  of  radiation  and  effec- 
tive temperature  of  the  sun,  306. 

Ross,  photometric  observations  upon 
the  corona,  254. 

Rotation  of  the  sun  demonstrated 
by  displacement  of  lines  in  the 
spectrum,  100, 101. 


Rotation  of  the   sun,  peculiar  law 

of    equatorial    acceleration,    138, 

144. 
Rowland,  H.   A.,   concave    grating 

spectroscope,  70. 

diffraction  spectroscope,  67. 

list  of  elements  identified  in 

the  sun,  87,  88. 
—  photographic  map  of  the  solar 

spectrum,  78,  79. 

Runge,  spectrum  of  helium,  347, 348. 
Rutherfurd,  diffraction  gratings,  66. 

—  solar  photography,  47. 

SATURN,  influence  on  sun-spots, 
159. 

Schaeberle,  J.  M.,  theory  of  sun- 
spots,  186. 

photograph  of  eclipse  of  1893, 

248,  255. 

theory  of  the  corona,  271. 

Schemer,  Father  C.,  discovery  of 
sun-spots,  114. 

Scheiner,  J.,  on  solar  temperature, 
308. 

Schmidt,  theory  of  the  constitution 
of  the  sun,  229. 

Schott,  drawing  of  eclipse  of  1869, 
245. 

Schuster,  spectra  of  oxygen,  91. 

Schwabe,  discovery  of  the  periodici- 
ty of  sun-spots,  151, 152. 

Secchi,  classification  of  prominences, 
218. 

—  drawing  of  eclipse  of  1860,  241. 

—  drawing  of  a  sun-spot,  116. 

—  estimate  of  the  sun's  temperature, 
305. 

—  formation  of  detached  cloud-like 
prominences,  221. 

—  measurement  of  the  variations  of 
temperature  at  different  parts  of 
the  sun's  disk,  302. 

—  photographs    of   the    eclipse    of 
1860,  and  inferences  from  them, 
195, 196. 

—  solar  eyepiece,  58. 


INDEX. 


373 


Secchi,  thermopile  observations,  302. 

—  theories  of  sun-spots,  181, 185. 
Sherman,  observations  at,  166,  167, 

'205,  225. 

Siemens,  W,,  theory  of  solar  heat, 
318-320. 

Sierra,  synonym  for  chromosphere, 
192. 

Silvered  object-glass  for  viewing  the 
sun,  54. 

Slitless  spectroscope  applied  to  the 
corona,  261-263. 

Smyth,  records  of  rock -thermome- 
ters at  Edinburgh,  173. 

Solar,  constant,  defined,  296. 
—  value  of,  296,  324. 

Solar  parallax  (see  Parallax). 

Soret,  penetrating  power  of  solar 
radiation,  310. 

Sources  of  solar  heat,  311-315. 

Space,  temperature  of,  4. 

Spectral  photometer,  Vogel,  280. 

Spectra  produced  by  prisms  and 
gratings  compared,  72. 

Spectrograph,  the,  71,  76. 

Spectro-heliograph,  233-236. 

Spectroscope,  analyzing  and  inte- 
grating, 72,  73. 

—  automatic,  203. 

—  described  and  discussed,  61-76. 

Spectrum,  explanation  of  its  for- 
mation in  a  spectroscope,  61, 
62. 

—  diffraction,  67,  72. 

—  lines,  displacement  of,  by  motion 
in  the  line  of  sight,  97-101. 

—  of  the  corona,  257-261. 
a  sun-spot,  132-136. 

—  solar,  discovery  of  the  dark  lines, 
59. 

early  investigations  as  to  the 

origin  of  the  dark  lines,  60. 
Kirchhoft's  explanation  of  the 

dark  lines,  81. 
maps  or  drawings  of  portions, 

77,  78, 100, 134,  135,  167,  209,  210, 

225,  231,  258. 


Spoerer,  distribution  of  sun-spots, 
148,  149. 

—  estimate  of  the  sun's  temperature, 
305. 

—  formula  for  sun's  equatorial  ac- 
celeration, 140. 

—  peculiar  law  respecting  latitude 
of  spots,  157. 

—  recurrence    of    spots    at    special 
points  on  the  sun's  surface,  150, 
334. 

Spots  (see  Sun-spots). 

Stannyan,  Captain,  discovers  the 
chromosphere  in  1706, 194. 

Stephan,  law  of  radiation,  307. 

Stewart,  Balfour,  discussion  of  mag- 
netic observations  at  Kew,  154, 
164. 

uncertainty  whether  sun-spots 

raise  or  lower  terrestrial  tempera- 
ture, 172. 

Stone,  calculation  of  solar  parallax, 
23,24. 

Stoney,  J.,  suggests  carbon  as  chief 
constituent  of  the  photosphere, 
339. 

explanation  of  absence  of  he- 
lium from  earth's  atmosphere, 
349. 

Struve,  brightness  of  the  corona,  255. 

Sun-spots,  cyclonic  motion  of,  126. 

depressions  in  the  photosphere, 

128, 131. 

development  and  dissolution, 

122, 123. 

dimensions,  126, 127. 

discovery  in  1610, 114. 

distribution  on  the  sun,  147- 

150. 

disturbances  connected  with 

them,  120, 122. 

—  duration,  119. 

—  effects   upon  the   earth,   161- 
177. 

periodicity,  151-161. 

spectrum,  132-136. 

Spoerer's  law  of  latitudes,  157. 


374 


INDEX. 


Sun-spots,  theories  as  to  formation 
and  nature  of,  177-190. 

visible  to  the  naked  eye,  114, 

127,  128. 

Swan,  spectroscopic  observations,  60, 
80. 

Symons,  connection  between  sun- 
spots  and  rainfall,  175. 

Synclinal  structure  of  the  corona, 
264. 


TACCH1NI,  sun-spot  of  1883, 128. 
Tarde,  sidera  Borbonica,  115. 
Telespectroscope,  74,  76. 
Tempel,  drawing  of  eclipse  of  1860, 

242. 

Temperature  of  the  sun,  305-309, 
324. 

the  sun's  center,  331. 

sun-spot,  170. 

—  terrestrial    as    affected    by    sun- 
spots,  170-173. 

Teunant,  Colonel,  observation  of 
eclipse  of  1868, 197. 

Thickening  and  thinning  of  lines  in 
the  sun-spot  spectrum,  133. 

Thermal  rays,  groundlessly  distin- 
guished from  luminous  and  chem- 
ical, 285. 

Thermopile,  170,  298, 302. 

Thollon,  great  map  of  the  solar  spec- 
trum, 79. 

—  powerful  spectroscopes,  65. 
Thomson,  Sir  W.  (see  Kelvin). 
Tisserand,  formula  for  sun's  equato- 
rial acceleration,  140. 

Transits  of  Venus,  22-31. 
Trouvelot,  veiled  spots,  136. 
Tupman,  drawing  of  the  eclipse  of 
1871,  246. 

—  work  upon  solar  parallax,  24. 

ULLO  A,  Don,  observation  of  eclipse 
of  1778, 194. 

Uraninite,  a  source  of  terrestrial  he- 
lium, 346. 


VARIATIONS  in  solar  radiation, 
304,  343. 
Vassenius,     early     observation     of 

prominences,  193. 
Veeder,  aurora  period,  137. 

—  relation  between    sun-spots  and 
aurora,  165. 

Veiled  sun-spots,  Trouvelot,  136. 

Velocity  of  motion  in  solar  promi- 
nences, 225,  226. 

Venus,  influence  upon  sun-spots, 
158. 

—  seen  at  transit   before    reaching 
the  limb  of  the  sun,  255. 

Vicaire,  estimate  of  the  sun's  tem- 
perature, 306. 
Violle,  actinometer,  294. 

—  measure  of  the   sun's  heat,  293, 
294. 

—  value  of  the  solar  constant,  296. 
Vogel,  diminution  of  light  at  the 

sun's  limb,  280,  281,  284. 

—  effect  of  the  sun's  absorbing  at- 
mosphere  upon  his  total  bright- 
ness, 284. 

—  exposure  slide  for  solar  photog- 
raphy, 50. 

—  spectral  photometer,  280. 

—  spectroscopic  measurement  of  the 
sun's  rotation,  100. 

WARRING,  illustrations  of  the 
sun's  attracting  force  on  the 
earth,  41. 

Waterston,  measure  of  solar  heat, 
293. 

Wilna,  photographic  observations, 
52. 

Wilsing,  determination  of  sun's  rota- 
tion by  observations  of  the  faculse, 
140. 

Wilson,  Alexander,  discovery  that 
sun-spots  are  depressions  in  the 
sun's  surface,  129. 

Wilson,  W.  E.,  and  Gray,  on  tem- 
perature of  the  sun,  307. 

on  radiation  of  sun-spots,  170. 


INDEX. 


375 


Wilson,  W.  E.,  and  Gray,  diminu- 
tion of  heat  at  sun's  limb,  302. 

Winlock,  horizontal  photohelio- 
graph,  27. 

—  annular  slit  for  spectroscopic  ob- 
servation of  the  prominences,  213. 

Wolf,  magnetic  variations  following 
sun-spot  period,  164. 

—  periodicity  of  sun-spots  and  rela- 
tive numbers,  152-154. 

Wolf-Rayet  stars,  helium  in  their 

spectrum,  345. 
Wollaston,  discovery  of  dark  bands 

in  the  solar  spectrum,  59. 

—  measurement  of  the  sun's  light, 
275. 

YODNG,  discovery  of  bright  lines 
in  the  spectrum  of  the  corona, 
250,  260. 

—  disturbance  of  lines  in  sun-spot 
spectrum,  100, 135. 

-  double  reversal  of  D  lines,  135, 
208. 

—  duplicity  of  corona  line,  257. 

—  examination  of  basic  lines  in  the 
solar  spectrum,  93. 

—  experiment  showing  the   black- 
ness of  dark  lines  to  be  only  rela- 
tive, 81. 


Young,    observations    on    chromo- 
sphere lines  at  Sherman,  205. 

—  observations    on    the    corona   at 
Denver  in  1878,  239. 

—  observations       on      remarkable 
prominences,    216,   217,  221,    223, 
225. 

—  proposed  explanation  of  equato- 
rial acceleration,  141, 142. 

—  resolution  of  sun-spot  spectrum, 
132, 133. 

—  reversal  of  dark  lines  at  the  be- 
ginning of  totality  in  the  eclipse  of 
1870,  the  reversing  stratum,  82, 

—  solar  eruption  followed  by  mag- 
netic disturbance,  166, 167, 225. 

—  spectroscopic  measurement  of  the 
sun's  rotation,  100. 

—  sun-spot  spectrum,  132-134,  339. 

rf  ANTEDESCHI,  development  of 
L*L    the  spectroscope,  60. 
Zollner,  estimate  of  the  sun's  tem- 
perature, 305. 

—  spectroscopic  measurement  of  the 
sun's  rotation,  100. 

—  theory  of  sun-spots  and  liquid  sur- 
face of  the  photosphere,  144, 182. 

—  vibrating  slit  for  observation  of 
the  prominences,  200. 


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